Very cool idea!
The enemy design is very cool, and you have quite a large variety. It feeds pretty well into the mutation theme, which I like a lot. Overall the aesthetics are very well done, I have no feedback here!
Gameplay wise, I agree with other comments. It could've been cooler if the game was a bit faster paced. At the start of the game it feels a bit slow, since you can't move too fast and finding enemies takes a while. I could sit comfortably in one place without moving up until about 3k score, at which point I had to start running, and then I noticed how slow the player is compared to everything else. You can maybe explore the player's upgrades a bit more by allowing them to get upgrades that aren't only centered on your rifle, such as faster movement speed, more health, etc. It might even be cool to give the player character some visible mutations if they do get these upgrades!
In terms of sound, I feel like some sounds are great and others are a bit tiring. The "score" sound you have is a bit too metallick-y and bright, which tends to fatigue the ears, especially if played repeatedly. You could use some more "wooden" or opaque textures instead and less harsh transients for this sound! The music was pretty cool and I had fun with it.
I feel like you did a very good job with the game in general. With some more polish you could have a very addicting game!