I found your idea quite interesting. Introduced mechanics were interesting, especially wall dash and ball shooter! Cloner was unique addition but felt a bit too easy to dodge.
Of course some visual polishing, and sounds would help to build better atmosphere in the game, but since time frame was short I do understand that you had to make those sacrifices, and that is absolutely fine. In my opinion showing your idea is above anything in game jams, so well done.
For visual part I would suggest taking a look at color pallets. Simply picking random pallet and having all level objects use it would improve level look. You don't need to have amazing art or anything, just following chosen color scheme will make whole game look polished (since every different color in the game adds some constraint on the player to pay attention to that object and that makes it not look very pleasing).
Also, try to take a look at point filer for sprites in unity, it will make your sprites not blurry.
Some particles, hit animations, or simply on hit color change and hit sounds would also improve whole experience to the next level!
For level design, I can not say much since I am not that great myself on that subject. But it felt okay for me and I completed whole game, maybe some parts were easier, but I think it is better to make game too easy than too hard in game jam! Maybe the only thing would be to introduce every mechanic one by one with levels, so the player could get used to that.
AND do not put slime near the exit on explanation level! I was reading about that statue guy and accidently walked out to the next level and had to restart whole game to get back :DD
Also, put some cap on how many slimes you can spawn since it can get pretty laggy if you spam space a lot.
Overall, do not feel discouraged for my critique, these are just suggestions if you decide to continue. I think it is important to know where and what too improve, at least I know that these feedbacks helped me a lot in making better games! I liked your idea, and with some improvements it can really become a nice game. Good job and keep making games! Looking forward!