Very, very much agreed. It's too flat, repetitive, and there's no variety in texture or colour. All the levels basically look the same and the forest caves/ghost forest tilesets are all just edited off the original forest tileset. The forest world mock-ups look so much better than the final product.
I think the existing forest background used in the majority of the levels is ugly and needs to be replaced, and a few new backgrounds need to be made. Most of the levels look the same. It'd be great if the tileset was revamped and didn't look so ugly, but I doubt it's realistic to think that it will be replaced.