The core mechanic of this game is great. I like that different animals behave differently - some flee, some attack - and they move in different ways. It is definitely hard to kill some of them, but it is also more satisfying when you do.
The concept of the upgrade system was great, but it didn't seem to do anything noticeable for me.
One tip for you: If you are going to participate in future GBJAM jams, then note that there must be exactly 160x144 square pixels on the screen. This is not the case in your game. For example, the animals move one screen pixel at a time, instead of one "game boy pixel". The easiest way to achieve proper square pixels is usually to set up the project to render at 160x144px resolution, and then scale it up afterwards in a pixel-perfect manner.