First: OMG Scratch!!!!
Art: I liked that, even if it was a character from scratch, the walking, sliding, etc. were animated. There even were the little flowers, which made it seem very cute! However, I don't think the colors of the game fit very well together. I think there maybe were too many different flashy colors? Maybe you could have used a palette from lospec to help with that! (that's one of the few tips I know)
Levels and gameplay: I never understood what the potions did and why I had to gather them! Nonetheless, I liked that every level had its own difficulty. I didn't finish the game, but I checked in the source code, and all the levels seemed to include a different challenge which is really great! Now, I may not be the most skilled player, but I gave up at the level with the second checkpoint because it felt unreachable :( There were too many ennemies together and even if I did escape them, the checkpoint was in a place too well guarded, and they were too many together for me to kill without dying. Maybe the checkpoint could have been in a different place, less ennemies (particularly the closest one, that we cannot see immediatly when entering the level) or not instantly going back when we loose one life (but that changes a lot about the game!). I had a bit lag when trying to rotate the purple level with the two monsters for the first time, but I don't know what happened! I liked the fact that we could regain lives, it felt rewarding for reaching a turing point!
That's a long text, but I hope it helps, congrats on your plateformer and feel free to ask for precisions or more!