Planet Robobot true arena is too intense, mostly due to Star Dream's lenght and the extra phase doesn't help.

You could say yes for both, I guess.

Too hard isn't fun, and neither is too easy.

Yeah. It's contex-sensitive, but why not? Judging difficulty is quite hard, however. There's a fine, thin line between complaining/praising a specific difficulty and it resonating with the people.

You just became my favourite user just because of your username and profile picture.

Not sure about Mario 64. Tropical Freeze could use more lives and better checkpoints for those wanting it. I know i would want them. Donkey Kong Country returns on 3DS had more lives compared to Wii version. 

Edit: Even Souls games could give more soul drops and less penalty for dying. I just avoid commenting on Souls as fans are really defensive about the grind. 

2nd Edit: Oops didn't even answer the thread question. Yes, critics should mention the difficulty level of the game. Also random difficultry spikes that are annoying in later half of the games. Ofcourse, whether it is positive or negative would depend on the end user. 

More health and/or less damage from enemies or from falling, more checkpoints, longer jump distance...

Difficulty isn't a necessity for enjoyment, but the sense of accomplishment from beating something is certainly amplified by there actually being some resistance. Do yes, I think it is fair to criticize a game for being too easy, and for the record, I see at least as much criticism about games being too difficult as I see about games being too easy.

I think so. I imagine it's the same as picking your standard fiction book and discover it's written either as a technical manual for NASA engineers or makes use of the vocabulary of a four-year old child on Kindergarten. Difficulty might be subjective, but then, it should be considered reasonable by the average gamer or at least your intended public.

Both your answers are quite odd to say the least. What makes platformers work is that they give the player a controlled environment in which they have to accomplish certain tasks. Souls games are constantly criticized for not having difficulty options because it's easy for people to imagine a difficulty slider making enemies take more damage or have less health - as that has been popularized by games since the 90's.

Platformers never had this problem because platforming games have always been about skill checks that range in difficulty. The thing with platformers is that they are so meticulously designed around the skill checks that there hasn't been a popularized format of difficulty modes in them. So when you say stuff like "increase jump distance" or "add more checkpoints" we are forgetting the point of the platformer. Platformers usually have a difficulty select built into what level you play. Levels get harder as you go and optional levels unlock as you go.

There is really no difference between a Souls game and a platformer. Both benefit from the controlled environment the game provides. The difference is that you don't see platforming fans whining about issues because platformers are the most obvious form of a skill check in video game history.

Edit: Sorry, I don't know how I made so many typos in my OG reply!

Of course it's fair. The nature of difficulty, or lack thereof, has a major impact on how enjoyable a game is. Whether you like your games easy or hard, some important questions will always apply. Is it fair? Does the game apply consistent logic? Does it progressively ramp up, and if yes, does it do so without resorting to cheap methods? If these things are off in some way or other, chances are the game will either bore or frustrate you. Possibly both.