This is an adorable game, and I really want to go get a cupcake now. I love how the little characters get the happy rosy cheeks when they eat their cupcakes :)
I will say that the difficulty spike in level 3 really surprised me. The first two were quite easy, and then all the sudden the 3rd level required insane precision to complete. I only got 3/4 after multiple attempts, and that felt like it was primarily luck.
I think a fun puzzle element might be to have objects in one dimension that alter something in another dimension. So maybe there's a red button in the blue dimension that moves one of the red platforms to cover some spikes, idk.
My last nitpick here - while I understand how you used the idea of "only one," the game's theme really centers around this multitude of dimensions. Nothing about the experience feels limited in an "only one" kind of way, in fact quite the opposite. I would challenge you to rethink the design a bit (just for fun), to see if you could make the theme more relevant to the game play. Like what if there was this cosmic cupcake that could be in only one dimension at a time, so you had to have the characters collect it in a certain order as it bounced between dimensions?
Anyways, excellent job, I hope you had a blast with this jam!