After speaking about it with a bunch of people I agree that the red hills are too hard on the aesthetics. While at the same time the rock texture (or any other similar texture that clearly depicts the impassable terrain) is already having a similar function.
I would personally find something like this useful - just to indicate where the terrain is bumpy and to determine whether there is a slope. It is only visible when you are completely zoomed out.
It is a bit hard to see due to the resolution of the image, but once in-game it is well readable while not taking away too much attention.
edit: more screenshots
fully visible
partially visible
and gone
Another edit:
Excluding the hills, to keep them as bright as teeth