The first thing I notice is that the trees have much more detailed textures than the mushrooms. I'd add some texture to the muhsrooms to match the rest of the art style, plus give the mushroom more... volume? If that's the right word. Mainly the mushroom cap. I made a quick sketch to illustrate what I mean:
And for the night sky, I'd add more stars to fill out the blue space, but make them more spread out than the stars you already have.
In any case, I think you got a pretty nice style going. :) I like the way you've mixed 2D and 3d! (Personally, I think the grass looks good.)