What I liked:
I like the art style. The asset pack you guys chose looks great and I like how the plants and flies look even if they mismatch a bit.
I like the layout and design of the level. I am a fan of the design and the layout, and I like how compact things are. I think that contributes to a chill feeling.
I like the animations and the controls. I think the animation and control of the character feels very smooth and natural. The shooting feels good too.
Some things I think need improvement:
You need menus, start screen and credits. I know this is obvious, just saying,
You need more sound design. This is also obvious. I think having some chill music and some more sounds would really improve the game and the vibe you're going for.
In my opinion, you should flesh out the "game" more. By this I mean that the goal is unclear to me. I get that I have to defend the flowers, but I don't know for how long. I don't really have an idea of how well I'm doing. I also don't really understand the different types of plants or weapons. How can I win? I think fleshing this out more and making it clear to the player will significantly improve the game.
Beyond those things I think I would be fun to implement another type of enemy. This could add some more variety and interest to the game. You could even add more flies with slight variations. I think that would go a long way.
Overall, I really like the direction this game is going, and you guys have a solid foundation. Good work!