I think it needs a bit more space at the top and bottom as well, since it still looks a bit cramped. You can also make the mushrooms form a more dynamic pattern. Having everything line up like that makes it look static.
You also have some
banding in there.
You can cut down on the colors quite a bit and still end up with a nice and smooth image by eliminating similar colors and constructing color ramps that can share colors for anti-aliasing. Try to replace a color with similar colors. If there's little change, you have one less color to worry about. You can also replace two similar colors with a mix of the two if the difference is too great.
For the anti-aliasing, you can try to pick a color that almost fits in between two others and then adjust it or the other colors so that it fits. It's easier if you keep the ramps visible in a part of the image so you can see how everything fits together. That way, you won't end up adjusting a color in one place and ruining the anti-aliasing elsewhere.
Also, whenever you have large areas of a single color, you can mix other colors in there to break up the monotony.

I circled the girl with mushrooms to make it a bit more obvious that she's the center of attention. I also made her turn slightly so that the pose is a bit less symmetrical and more interesting. I also ended up toning down the shadows on the mushrooms a bit and darkened the stems surrounding her to lower the contrast of the background and make her stand out more. I've used 18 colors.
Hope that'll give you some ideas.
