Very cute game!
I like the pixel art and the mechanics are simple but well polished.
The aiming mechanic is not used since enemies always come from the same 3 directions. It could have been WAD or left/right/up arrow to shoot and it would have been the same and actually spared us some wrist pain you get later in the game from clicking back and forth at high speed.
Or instead, let the parachute enemies to fly at you from multiple directions.