My number one critique ... I want more! This game was a blast to play and I always love when I get the chance to use a gamepad in a jam game.
Positives: I loved the art style. I connected with that round little spaceman immediately and wanted to play more because of it. The animations overall were really well done, great polish for a jam game. The music selection and variance was perfect. With just two enemy types, a few obstacles and a moving platform you designed some fun, tricky portions to navigate. The checkpoint animation was fantastic and I think you had just the right amount! Loved the sound design, but think just a couple more sfx would have gone a long way.
Improvements: It was so polished, that the areas where you didn't add juice it made it seem really off. The particles on the blue blob when you hit it were nice, but an audio cue would have completed it (especially since you got audio feedback on the floating spike balls). Short time to dev and all, but maybe you could have reused the audio from when the boss spawns the jelly creatures, just alter the pitch. The floating spike balls would have had that extra touch of nice with a small shine animation. Just something more to make it feel alive. I know it's a short dev time, but I really wanted some pickups. Not powerups per-se, but there were times when I would think, oh, I bet I'll find a collectable over here, but there's nothing in the game. One idea is that since you suddenly have a life bar in the boss battle, maybe you could have spread out little life capsules in tricky to reach or hidden areas, that way there's a reward for collecting them in that you're stronger when you fight the boss, but also you don't have to get them if you're good enough in the boss battle. I'm one of those weirdos who uses the joystick and not the d-pad in platformers. I think you could have given in a little more buffer space on the stick. My character would stay turned right often enough that I switched to the d-pad and had no problems. As awesome as the art and animation is, I really wish the jetpack was animated, even if it was a simple two sprite swap. You spend so much time jumping and hovering, it seemed off that the flame was static.
Slynrd covers a really simple way to animate flame movement in his blog here: https://www.slynyrd.com/blog/2020/12/14/pixelblog-31-shmup-sprite-design
He also has a good one on items. I think the shine effect would have gone really well with the floating spike ball of death.
Overall: I loved this game! I see myself enjoying exploring even more of the world, maybe gathering some standard metroidvania powerups, a double jump for instance, or you could lean into weapons like Cave Story. Really fun project and I hope you keep building on it.