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.)

I would suggest having everything be the same art style as the giant mushrooms.

I might echo the words of a couple of people above such that it helps to make everything feel the same artistic style. But that style can be in whatever form. Consistency is key! As such, it's important to choose something where the amount of work you'll have to do to accomplish that takes less effort. 

From my own experience from playing around with placing textures, people's eyes need to see slight variations; even if their brain doesn't see them.

A couple of ways to do this is to:
Generate a bunch of patterns with the same general look and reference each variation of art asset for each object of that type
Introduce some irregularity to a repeating pattern with sprite masks and/or translucent pixels (that is — colours with alpha-channel values to let you see through them).