Perhaps rounded is not the correct word. What I mean is to make the bottom of your mountains not cut by a straight line. You would either need to make 5-10 mountains to use as "stamps" or probably easier to just make each mountain unique and have the shadows come a bit below the outline of the mountain in a few places. It kind of needs a bit of extra definition to make it look a bit more 3-D ish(unless you are specifically going for a flat look).