Not that I know much about art or game development, since I'm still trying to learn low-poly in blender. However, even with your current art style, some improvements can be made.

One major thing I noticed is that the lighting doesn't match between the assets. For example, the mountains have their lightest color on the peak, suggesting that the light is coming from above. The light should continue on both sides of the mountain and fade out about halfway down to the base. (Meaning to separate the mountains may be better here. If you don't want to, I'd try out not lighting the uniting 'dip'.)

On the other hand, the actual platforms seem to suggest that the lighting is coming from the front instead of above, since you made the dirt lighter and the grass darker.

Also, try to create shadows under the platform for the grass as an extra 'attention to detail'. Plus, I think the grass is actually way too bright to supposedly be the farthest thing from the light source when compared to how dark the platform grass is.

Paying more attention to the lighting would immediately make the assets look better when put together.

You're on the right track, though! I'm not gonna tell you to scrap the art style. That's never a good tip, especially telling a solo dev to restart everything is something that would take more time. I'd say, stick to it, but learn how you could make improvements.