I liked the main mechanic of the game, and liked the polish you've added in the game.
But I think you could add more polish, since you went for a simpler gameplay/art style for you game.
For example: make a simple animation for the bullet that scales up and down, giving a impression of livingness on it, add a trail to the bullet, so it makes a cool line, add some UI tween and polishing as well and so on...
A goal score, just to show some congrats message would be a nice addition too.
Overall I liked it!