I really don't understand how it's showing. it's selling well but other 2d marios have sold much better, and it's really starting to fall of the charts. The main problem is it's thing is about being creative but many of the levels  feel like something made by small indie game they have the budget to take 2d mario into a whole new level, but they chose most likely a very small budget on it.

It's still in the top ten after 7 months after release and has sold as about as much in under 5 and a half months as NSMBUD sold in 4 years.

And it's not low budget at all; a huge amount of polish has gone into it and we know from Nintendo that they tested thousands of Wonder Flower ideas. Extensive prototyping and the level of refinement Wonder shows takes a lot of man-hours, and devs have to be paid for all that work. Development began in 2019, so it had at least four years of dev time.

lets see where it ends up but almost every major franchise has broken records on the switch while wonder won't even be the 4th best selling 2d mario. comparing NSMBU a port of a 5-6 year old game when it came out is not really fair.

As for the wonder seed most times they are super easy to find, with 25-35 seconds  simple gameplay, it's like saying the puzzles in mario are pushing new boundaries not really it's something made to be very simple and cute. 

There are 4 Mario platformers on Switch. 3 of them are entry friendly fixed camera games with up to 4 players. There is a lot of choice. Not even talking about the numeral NSO options for 2D Mario. There is simply a lot of competition for SMW.

Don't underestimate how much playtesting and refinement is needed to hone dozens and dozens of unique gameplay ideas. What looks simple to the player can take an immense amount of work behind the scenes. If it was as simple as you say the game could've been made in a matter of months, when in fact we know it took 4 years.

It really depends the size and budget of the team. That plays a massive role as well.

“The singing piranha plant” level and all the praise it got on the internet represents really well one of the main issues with 2d Mario these days (overly focused on “game mechanics” is another problem). This idea that the central point of 2d Mario games must be whimsical/silly is completely wrong. Much like the Mario movie, 2d Mario is an adventure in a mythical world. Does it have silly moments? Of course it does, but it should not be the main focus of the game/movie.

The release of three Mario titles (Mario v. DK, Peach Showtime!, TTYD) certainly isn’t helping.

There's one constant in Nintendo games: franchises can morph at any time. You know all the examples of that.

I think that if Nintendo hadn't released NSMBU Deluxe and had released Wonder in the Switch's first years, it definitely would have sold as much as Mario Odyssey by now