Nice shooter. Here is some feedbacks:
The art is fine (with some 2D lights and postprocessing it would look cooler in no time, and will help diversify you environment too !).
I like the humor of the game.
The UI with the saturated color need some tweaking, and also change the default unity font.
Something to view our progress in the dungeon would be great.
I like the bosses too, not too hard (at least the firsts). The transition is a bit abrupt
Some 'cheap' background could help too (but just some black will be better I think)
Overall good work !