Fun little idea, but I feel like it could be executed better.
1) The art style should be the same. If you are using pixel art, make everything pixel art
2) You are too OP, I found my self going right and left and pressing the shoot button. I didn't have to do any work. You can fix this by either decreasing the speed of the player or increasing the health of the enemies
3) The boss fight is too easy, it acts like a normal enemy. The boss should have some kind of attack which differes from all the other enemies.
But, overall good game!