I'll edit this as I think of more feedback.
The toad house minigame where it shuffles items could be better. Normally it isn't all bad since if you can keep up with the item you want, you can get it. But in this, the items are just teleporting around. If you can, I think giving the hidden items smoother movement would make it alot better.
This game seems to not feature coyote frames. They're always nice to have in a video game. yeah that's pretty much it what else can I say
I think the player's sprite on the overworld should always be their small sprite, but with the colour palette of the powerup they have. The big sprites with everything else on the overworld feel kinda weird.
Another thing that imo feels too big for the overworld is the castle sprite. You could make a smaller sprite but I wouldn't mind if you just shrunk it.
I think it'd be cool where if you collect a powerup you're already using it gets put in your reserves. If you feel this is a bit overpowered, some coins to help you get a reserve powerup would also be nice.
Other than this stuff, I'm really enjoying this game! Pretty good difficulty level, nice level design... nice! Looking forward to finishing this comment so I can get back to the game!