This is a great entry! The atmosphere is phenomenal - I actually was going for the same kind of mood in my game, but you did it in much better. The pacing and the atmosphere is pretty great!
It's actually hard to write this review because rittz (the comment below me) pretty much echoes all the criticisms I would have made. The snail on a tiny platform in particular came up a lot, and it was pretty frustrating to handle. This is especially because of the hit box of the character, which seems circular and would mean it would fall if it was on the corners of platforms.
I understand also if you weren't aware, but CTRL as the control for the shell was bad for mac - ctrl + arrow key would trigger switching of screens, so I had a lot of trouble maneuvering through the bubbles. I also feel like the slowdown was largely unneeded (or could have been reduced).
Overall I do think that you knocked the mood out of the park, and it's a great solid entry.