Here's my rating:
The gameplay was fun. There's some really good variety in the enemies that really brings the game alive. However, it is rather unbalanced. The green shooting enemies are WAY too powerful, but the other green enemies that just float down the screen are completely helpless. The score powerups are just random free points, and the health powerup gives you a completely full health bar. Also, if you have 3 or more enemies attacking at the same time, you will die instantly because there's no invincibility time when you get hit. My rating: 4 stars
It fits the theme. It's a galaga clone. Anything along those lines will fit the theme. My rating: 5 stars.
The aesthetics, although simple, are not that great. The edges are really sharp and ugly. Basically, antialiasing needs to happen for everything in the game, including UI and game sprites. I recommend playing on 1x resolution because it helps with that issue. Also, the similar color pallet for some of the enemies prevents instant recognition of those enemies, especially since the rest of the enemies rely on color and shape to convey who they are when they enter the screen. My rating: 3.5 (rounded to 4) stars.
This game really needs some music. It has some sound effects, and they are good, but they only play on half of the things they should play on. Basically, you ran out of time for the implementation of the SFX. I understand. My rating: 2.5 (rounded to 3) stars.
Overall, I like the game. You focused a bit more on the mechanics than on the art and sound, which did benefit the game and its fun factor, but the slightly ugly art and the lack of good sound really put some bullets into the game. But hey, you submitted the game, which means that your score is way better than the 0 stars you would have gotten if you hadn't submitted. My rating: 3 stars.