Hi,
The most obvious issue with the game is that it is very repetitive, and too easy (in a few rounds, you already have enough money to buy attack upgrades and one shot everything). Moving enemies, new enemy types, smaller ones to challenge click precision, enemies that shield the ones around them and prevent the player from killing them, etc. There's a lot of room for more content which would be cool!
Another thing is, if your game is very simple and played only by clicking, that is fine, but then, each click needs to feel great. Here, clicking on an enemy just decreases its health bar and will spawn a sprite on death. What I would expect is that the enemy shakes on click, explodes on death with a sound and a screen shake, stuff like that, so that I really feel like I'm powerfully bursting enemies with my mouse.
That reminds me of that video from Jonas Tyroller, in which he works on a simple button and tries to make it fun to click on it:
Anyway, all of these are easier said than done, what matters is that you made a game that can be easily extended on, so I hope you'll spend a bit more time on it in the near future! And I'll be happy to play it again if so :)