No, she is not.  There is just too much evidence against it.

 In TLJ, Snoke was playing her the whole time, spanked the side of her head with her own weapon, ripped Resistance secrets from her mind and left her at Kylo’s mercy.  Kylo then had to save her because she was having trouble with one member of Snoke’s Honour Guard (the rest were chopped down by Kylo himself). 

TLJ was punctuated with her failings.  She almost fell to the Dark Side during her first formal lesson in Force Training, needed a lightsaber to beat an old man armed with a stick even though he had closed himself off from the Force, and she was such a disappointment to Luke that Yoda had to talk him into re-joining the fight.  Her efforts to pull Kylo back to the Light resulted in him choosing the Dark, and despite having a head start on him, was the last person to show up at Helm’s Deep, too late to even participate in the battle, let alone turn the tide.  None of this would have played out as it did had she been a Mary Sue.

All she accomplished was the opening of two doors: moved some rocks to get a handful of soldiers out of a cave, and then let them onto a spaceship.  Of all the heroes in the movie, she was the least accomplished.  She had nothing to do with the bombing of a Dreadnought (a class of ship the Resistance had never managed to destroy before), the infiltration or destruction of Snoke’s warship, or even the Resistance’s Hail-Mary sprint to Helm’s Deep.

Hell, the movie even said that she wasn’t anyone special.  Had the Force not chosen her to be a balancing agent for Kylo, she’d still be trading scrap for food and fending people off with a stick.  Even as a pilot she isn’t noteworthy.

While Han Shot down Darth Vadar’s Custom TIE, (despite being a petty criminal against an ace pilot combat veteran), evaded a squadron of enemy starfighters in an asteroid field dogfight, blew up the Death Star 2.0 and aced the timing for a hyperspace jump through an impenetrable forcefield...

Rey barely got the Falcon off the ground, required help from a trained soldier who was specifically familiar with the enemy’s equipment and tactics to barely evade/defeat them in familiar territory.  Even Luke did better in his first combat flight, blowing up the Death Star as the sole survivor whereas every Rebel combat veteran was shot out of the sky like it was a matter of routine. (And they were all killed before Obi-Wan had to remind Luke to use the Force).

If you’re going to lower the bar so much for her to fit your idea of a Mary Sue, the number of others clearing that hurdle would be so numerous that the entire concept would lose its special status among things worth complaining about.

She fits into the Chosen One archetype a lot better than the Mary Sue concept.  

1. Han didn't "shoot down" Darth Vader's custom TIE.  He shot down one of Darth Vader's wingmen.  Vader's other wingman flew into Vader's TIE which crippled it and sent it spiraling away from the battle. 

2. By the way, Han Solo wasn't some petty criminal out of his league against combat veterans.  He was a seasoned smuggler and expert pilot in his own right. 

3. Also, Han Solo didn't blow up the 2nd Death Star.  He brought the shields down on Endor so that Lando Calrissian and Nien Numb could lead a squadron of X-Wing to destroy it.  Luke was not the sole survivor of the first Death Star battle.  I don't know if any other X-Wings fought in the battle that didn't make Death Star trench runs.  But for sure, one of Luke's wingmen, Wedge, pulls out of the trench and survived to participate in the 2nd Death Star battle. 

4. (...painting 2 Death Stars on his X-Wing taking credit for both kills) 

5. Also, it's established in the film that Luke piloted a T-16 Skyhopper on Tattoine (he plays with a model of it early in the film), and that he has experience bullseye targets (womprats) that are no bigger than 2 meters in size. 

6.  The fact that Obi Wan coaches him through his Death Star killing shot is bigger proof against him being a Mary Sue than for.

Nearly $600 million domestic (far more than AotC, RotS, and Rogue One made lifetime), and $1.3 billion globally isn't what I'd call "faltering." That's a massive success by any standard. Failing to replicate the success of TFA is not a strike against TLJ. TFA clearly over-performed, and nothing should be expected to replicate it. Kinda like how we shouldn't expect any console to replicate the success of the PS2, or a handheld to replicate the success of the DS.

I pretty much agree with what you've had to say in this thread. I really don't have much more to add. There really isn't anything miraculous about Rey. There's nothing about her non-Force abilities that cannot be explained by even a cursory examination of her background. While the movies don't fully touch on everything, more is explained in ancillary materials like novels (Before the Awakening mentions she regularly uses a flight simulator program she found, which combined with her obvious familiarity with the Falcon in the movie showed that she wasn't any more wet behind the ears than Anakin or Luke were when it came to piloting). Her successes make sense in context, and she has failed on multiple occasions and clearly exhibits shortcomings (e.g., aggression, recklessness, difficulty resisting the Dark Side). As for her Force abilities, well "There's been an awakening." Both Rey and Ben are part of a new generation of powerful Force users, and even that kid at the end of TLJ casually uses the Force to pull a broom to himself, presumably despite lacking any real training. It just came naturally to him.

Rey isn't some flawless character. The idea that she's some sort of Mary Sue is a huge exaggeration that relies on a lot of jumping through hoops to justify (e.g., "Kylo being shot in the gut by a bowcaster, taking another injury from Finn, and being an emotional wreck after killing his dad on top of that doesn't count in his duel with Rey because reasons. She should have lost, period!").

The thing about internet backlash is that a vocal minority can be very vocal and very very hyperbolic in their rhetoric, but still be a tiny portion of a fanbase.

It's impossible to objectively measure. We honestly don't know how much TLJ would have made had it not rustled some jimmies. Nor do we know what Disney was expecting out of it. So far, they haven't commented. But at the end of the day, whatever goals TLJ haters have set are entirely arbitrary. Some people on this forum are now acting like the second-month gross is somehow more important than the first month or the total lifetime gross, even though second-month grosses are always a very small portion of a movie's lifetime gross. Others have suggested that TLJ needed to be well north of $700M domestic, if not over $800M, even though such numbers are extraordinarily rare. But thinking that it had to be closer to TFA than to, say, Rogue One is to me rather unrealistic.

TFA clearly over-performed when compared to every other major film this century. After just two weeks in the box office, it had already grossed a whopping $652M ($669M adjusted to 2017 ticket prices), eclipsing the lifetime grosses of many other successful films in recent years, including every Harry Potter film, every Middle Earth film, every Spider-Man movie, every Pixar film, every Batman movie except TDK, and every MCU film except for The Avengers (though it had passed The Avengers by its 17th day), among others. A week later it had risen to $770M ($801M adjusted), making it the third most successful movie of the past 30 years after Titanic and The Phantom Menace. Put simply, TFA was pulling numbers in its first month far beyond what any other film in history did in their first month. While it had proportionally weaker legs than the other non-Star Wars films on this chart (more on this in a bit), thanks to its obscenely strong first-month gross by time it left theaters it had become the fifth most successful movie of the "Blockbuster Age" of cinema (only A New Hope, E.T., Titanic, and Jaws sold more tickets during their original runs), and the most successful by far of the 21st century to date. It also did overseas numbers that were well in excess of what was normal for Star Wars in the past.

These are clearly anomalously high box office returns, and thus a goal no other film should be expected to replicate or even come close to. Only 13 films in the past 50 years made an adjusted gross of over $700M in their original run, and of those only six grossed over $800M adjusted. TLJ will likely end somewhere between $620-630M domestic. That puts it at #8 among all movies released this century. Could it have done more? Perhaps, but I doubt it would be by much. Given that TFA and RO made, respectively, 88% and 90.6% of their lifetime gross in their first four weeks (making both noticeably more front-loaded than other big 21st-century blockbusters), we could extrapolate that TLJ "should" have grossed between $640-660M. So, it will likely underperform slightly due to somewhat weaker legs, but we're talking making 3% to 6% less than it "should" have. Now, some people might want to extrapolate from the first week or even opening weekend, but I think some people would want to do that just to make TLJ's performance seem worse than it is, because first-week(end) grosses as a percent of lifetime grosses can vary wildly even within a single franchise, and they can pick an opening weekend that's a small chunk of the film's lifetime gross to make their point. There are any number of ways one can use facts in a misleading way to downplay a film's success.

Point being, using TFA as some bar for TLJ to reach or at least get close to is unrealistic. And saying that TLJ was somehow a box office disappointment is reaching.

Saying that particular scene could have been written better, or that a particular outcome to a conflict could have been better from a narrative standpoint, or just not liking a particular outcome, is a lot different from claiming that a character is a literal flawless Mary Sue. I've watched TFA quite a few times already, and TLJ twice, but it never once occurred to me that Rey was a flawless character. She always seemed pretty flawed to me.

With JJ back in the director's seat, I'm sure Ep. IX will be the "safe" film, kinda like how TESB was the OT's "risky" film and ROTJ was the "safe" film full of straightforward action-adventure. And like how ROTJ performed better than TESB (though not by much), I'm sure Ep. IX will do even better than TLJ (though still nowhere near as well as TFA, because it'll probably be a very long time before we see a box office juggernaut like that again). In fact, if Disney does think the concerns of the vocal anti-TLJ minority on the internet is worth addressing, we probably will see everything Star Wars play it safe from now on to avoid ruffling any feathers.

If you personally think she's a lame character, that's fine. I can't make someone like a character that I like. But I can call foul when someone acts like a character is objectively a Mary Sue when such an evaluation is A) largely subjective and B) requires a lot of jumping through hoops to justify. I honestly feel that some people are just hating "NuWars" for hate's sake, and finding whatever post hoc rationalizations they can to justify their hate.