Fun concept for a puzzler, but having to stand still for a second before transforming really ruins it for me, since press "d" a second to long? failed start again, stand to close to the guy? failed try again, and then when you do know that this run is dead you can only wait until the guy slowly walks into spikes since there's no reset key
The puzzles aren't bad and the concept is neat, it's just that this single nitpick does a lot to slow the game down
Regardless, you've still done a very good job, the transformation animation is amazing, the music adds so much without getting in the way and the puzzles are easy to read and figure out what you need to do
and as a final level design nit pick, maybe add mushrooms in the overworld before the player can add a mushroom on top of their head? that intro was a bit unclear, it would've taken a while to figure out if I didn't have experience with other games with bouncy mushrooms