I actually had tried your game prior to your post! Really cool take on flappy bird concept.
1. The separation from the killing mechanics from the background/world. It was difficult at times for me to understand what I was running into to die .
2. I would recommend to increment the players score for every second they stay alive, and have shooting an enemy be worth like 3 or 5 points. this gives the player a reason to want to try to live as long as possible, but at times risk trying to kill an enemy for points!
Good work here!