initial impression viewing this is pretty good
+1 tell us more about the gameplay.
ramblings...
the centre rock feels to regular like a perfect cylinder, i'd prefer it you made some deviations in the form such as a large shape connected and 1 or 2 smaller shapes around, just to make it look more natural.
don't shade the sun as mentioned. you can add some corona around the sun by shading the sky a bit but the sun itself looks like it should be pretty bright.
i would also avoid yellow for the sun or desaturate it so it doesn't compete with your sprites for attention.
atmospheric perspective of clouds (fading to sky/air colour) could use some balancing.
middle cloud hues not really doing any favours imo and the back one should have a bit more atmosphere (more faded to sky).
if you're in something like Gimp or Photoshop you could use a cheap cheat by cutting these clouds to a new layer, fill behind with sky then simply reduce their opacity.
then you could look at shifting the hues of the layers as desired.
there are more advanced 'cheats' but this is the simplest I can explain.
u have a light source but u don't rly ur shading doesn't use it.
the mountain on left for example is lit from left and sun is on right.
left side of nest at front should darker than right, should cast more shadow on grass etc etc.
remember that light and shade are your friends.
u don't need that much texture all over the grass for example as it makes it noisy and contrasty which again attracts the eye more than you probably intend.
instead you can suggest/imply texture by varying the pixels along the line between 2 bands of colour to create grass blades, then come along after and add a few patches of other blades and highlights.
with grass colours, try blending in a bit more yellow as it gets brighter so the yellow blades don't stand out so much.
general rule is things get bluer and usually less saturated in the shade towards yellow in the light.
i would avoid that straight brown ramp u have in the rocks now.
try desaturating that section then subtly blend in a few different hues as brightness increases to make it more realistic and interesting.
in rocks like that it also makes sense to have say 2 different ramps and vary the rocks a bit as if composed of different minerals (so you would create another ramp with hues that are shifted from the first one).
since the light source is kind of behind you would also want to consider using ambient lighting from the sky to further bring out the forms.
EDIT: just noticed from your post history you might be using a limited palette?
if so i wonder why as this can be so restrictive in art of this scale.