My commentary is mostly about the Gameplay. for graphics and sound, see in the other sections.

I'll start off with the positives. I like how simple the game play is. I like that the camera doesn't scroll up and the level design is very concise. I like that Toad's moveset is very simple. Mechanically the game is fine, though the lack of midair mobility can be annoying. Knock back can also be annoying.
The switch blocks being a central mechanic is also pretty fun, I like how the challenge with the switch blocks progressed as the game went on.
I really like how the game progresses for the most part.

Sometimes platforming can be annoying due to the lack of midair mobility + enemy spawning. Moving platforms and enemies move way too fast sometimes. Riding a moving platform and jumping into a bullet that you couldn't predict would be in that place at that time became frustrating. (Sewer level) There's a section where a piranha plant spawns where you couldn't have predicted it in Chapter 4 as well

Some levels just don't do a good job at teaching players the mechanics, specifically the raccoon suit level.
I really liked how the raccoon suit controlled, but the first section after you get the leaf does an awful job of teaching you how to use it. There should be a safe environment where the player can experiment with how the raccoon suit works, I still don't have a good understanding of what's the most optimal way of hovering and landing on the goombas. It's just not clear how to do it right

And then, after that, the raccoon suit mechanics are thrown out, instead now it works completely differently. I wish that there was more flow to how the mechanic was taught to the player and then used in difficult platform sections before you completely change how it works in a flight section. The flight section was also kind of unforgiving and, why would the player ever think to fly downward at the end?! Put arrows at the bottom of the screen rather than on that block. I think it would've flowed better if you had the flight section be the last section of Chapter 4. Also add more checkpoints.

I think Chapter 6 does a pretty good job of teaching the player how to use the sword and there's some really cool puzzle solving with the switches. I love how the difficulty ramps up in this level, but there really needs to be a checkpoint at the part with the black smoke. It would make the level so much better.

There's a few inconsistencies with how mechanics work in the game. For example, Paragoombas have inconsistent patterns. Some loop forever in 1 direction, some hover up and down repeatedly, some are move really fast for no reason. Some spinies fall off the platforms, some don't. Why not give each type different graphics? It'd help the player remember and know what to expect. Maybe Galoombas only hover up and down and Goombas loop indefinitely. The way the raccoon suit works is also inconsistent, maybe you could've explained the flight section by giving Toad a P-wing.

Overall I feel like the difficulty is pretty fair though, I just think that the level design could've been more thought out. I really love the switch block level where it's dark and how that level's difficulty ramps up.
I also really like the Chapter 5 level where spinies are introduced, though there's a Piranha Plant that's extremely unpredictable.