Very cute and addictive :) Despite the simplicity - it was quite satisfying, maybe due to that popping sound :D
The upgrades and game-play is a bit too passive - after level 3 or so - I didn't even move the mouse and just waited for enemies to die :) Even in the last level, I didn't have to move, which kinda removes any challenges.
Overall - great submission, especially for the first one, and kudos for Godot :)