This is wonderfully cute and generally plays like a dream. Mechanics and controls are introduced well!
It seems like corner collisions are slightly buggy. It doesn't impact game-play much, thankfully.
I wonder if just having pressing down as going into ball mode and up to leave it as good enough? Haing ball mode on Z was pretty confusing I found as I'd often accidentally hit it when going to jump. Maybe I'm just clumsy, though.
That's all really, I don't have a lot to suggest cause what's there is mostly amazing.