Wow this is awesome! Especially for a first jam game, congrats! Love the level design and art direction, and easy to understand controls and tight, somewhat challenging obstacles. I think you nailed all that stuff.
One thing about game jams I've learned is that it is never hurts to provide a way to prevent locking content for impatient players. Now these are some pretty long levels for jam standards so when a player dies and goes all the back to the start? That can be very discouraging and turn them off from trying again. I think a more charitable checkpoint system and a level select would help this tremendously. But aside from those small things, this is still great! Fantastic job!