I don't think you need anything on the hills if you have a lot of those midground cliffs around (like in the top screenshot). It would just be busy.
Depending on how detailed you want them, consider making the colour transitions have a bit of grass texture to them. Some small vertical strokes would be enough, since the hills are far away.
The rounded hill shape looks very unnatural and cartoony, they look more like barrows than hills. That's not necessarily a bad thing, but I thought I'd mention it. Real hills tend to have a less hemispherical shape, with clearer ridges and less symmetry, so that might be something to work on if you want a more realistic look.
If you keep the smooth, dome-like hill shapes, it might be good to make the shadows smoother to match. The shadows imply an irregular surface with lots of bumps and troughs, but the hill silhouettes imply a smooth, featureless surface.