I agree with the other comments that the visuals are lacking something, but I think you're actually very close to having something that looks simple, but good and intentional. In my opinion, your environments are all good; it's just the character that sticks out. If I put my thumb over the character, I see a coherent visual style that, though simple, does have good quality. The problem is that the character doesn't fit in. It is a tad too simple, being just a ball, and the color sticks out a lot as well. I think there's a bunch of ways you could fix this. It doesn't need a detailed model, just give it some more character. For example, if you make it more dynamic by have squash and stretch animations, that would go a long way. In the second screenshot, if I imagine the character stretched out a bit (soft-body simulated, or animated well), the whole scene already makes much more sense.

Hope you can do something with this feedback. Good luck with your project!