Is there a term for this sort of graphics?
If the terrain is indeed represented by a 3D mesh as it looks like in the images, then I'm not aware of any specific terminology (and I dont believe there might be one).
I could say it was similar to a faceted shading model, but shading looks very rudimentary and lighting computations might not exist at all.
In case you intend to replicate these looks, I'm afraid you already hinted how to proceed very low-polygon non-textured meshes and no lighting.