Great job! First things first, the art and music, literally perfect. Love the art style and honestly the music kept me playing for much longer than I would have otherwise! Bravo!
A few small notes for next time though:
The speed of the enemies is very slow, it makes the game extremely easy. I racked up 30k points from killing enemies and got bored so I stopped killing them and just ran around the map, seeing how many I could get to spawn. I did that for about 2 or 3 minutes before I jumped in a huge pile of enemies to end the game.
Also, when it comes to reloading I was really confused at first. After I pressed 'R' I had no way of knowing if I was reloading or not. A simple sound or animation or progress bar above the player's head would be super useful. I also noticed that you can't reload unless your gun is 100% empty. Not sure if this was a bug or a feature, I just found it a little odd.
I think all the enemies take too many bullets to kill. The slime should take 1, the bat 2, and the spider 3. Right now, they're all just a little too beefy. I would make their health is much lower but you spawn a lot more. They all also kind of feel the same, just slowly moving at you. You should make it so the bat moves super fast and does a little damage, the spider moves slow and does a ton of damage and the slime is somewhere in the middle of both. Just to add some variety.
All in all, I think the game is awesome! Again, knocked it out of the park with music and art, fantastic job on both fronts there. I hope you had fun on your first game jam! :)