Very cute game, love the undertale stuff. Since you asked for feedback I will address everything I thought about while playing the game.
I like how you did the theme in a very subtle way (except the chair part lol which was very, but it is nice that creates this cool contrast).
The controls could feel more responsive. Like you should be able to dash as soon as you finish dashing (and you should probably also add input buffering to it). though tbh the delay saved me a feel times lol. In this matter adding a cooldown bar to the bomb and the shield could be nice too, to show when you can use them again.
Talking about the bomb and the shield, it should be more clear their effect. tbh i got through most of the game without using bombs because after experimenting with it a bit i couldn't figure out what they actually did, and throwing them heated you up so felt a bit like a waste. Maybe you could make so the snails get dizzy for a tiny moment after taking a bomb or the bomb heats them up, or something like that. Also you should make more clear the shield is actually stopping you from slowing down in the mud, either make slowing down more abrupt so the shield effect is more evident, or/and make so that the mud under the snails stop having that movement visual effect under them when the shield is activated as a visual indicator. Also it is important this for the bomb, make so that there are clear visual and audio effects for when they took the bomb vs when they defended it.
About the music, you can not go wrong with undertale music lol. Though I must say you should turn the default volume down a bit. This isn't really an issue as it can be turned down in the settings, but the music is really loud in the default settings and that might be bad.
Also be careful with using assets and characters from existing IPs, though I hope this will not cause you any troubles, and it won't probably, but depending on the rules and their interpretations it could disqualify your game, what would be sad.
But overall congrats, you did good job.