Yesh, I think the completely covered hill is too twee and the folds with the gathers are off-putting. The edged hills are nice, though. Might want to try felt, as Doug pointed out, or a less bright shade of green.
Hi – seems a good idea to make the hills modular (stackable), but I don’t think the covering material is working because (a) it causes puckering when wrapped entirely around the base and (b) the contrast with the brown textured contour line just seems too strong.
I like the texture visible on the contour line and suspect that an easier look would come for apply a gentle scattering (to give patches of say a 25% covering) of sand and flock over the entire feature, including the contours, so that the contours are loosely melded with the flat surface material. The base material that you have left visible on the contours seems to have a great texture to work with and the grass mat covering takes away, rather than adds to that.
I have some hills that I would describe as too grassy and I intend to rough them up a bit, so that some added texture will catch the light better. I even think a bit of dry brushing with another tone might help, but I still want my hills to have an overall effect that they look similar to each other, so that my terrain feels a bit more ‘knitted’ together.