You would have to create it in a 3D modeling package and import into Unity.
Besides the terrain generator (which cant be used on mobile devices anyway), Unity only has primitives (cubes, planes, spheres, and such) for prototyping.
Change the opacity (strength) in the terrain editing, so that it makes it flatter, also, make sure the height map is VERY low, then smooth it out, then for the actual texture on it, find one online that looks cartoony, apply that to it, then for the trees, you could do either in unity or in C4D, just make a cylindar, add a sphere on top, then put the sphere in the leaves, and the cylindar in the trunk, then apply a cartoon texture, then put it in the correct folder after making that a prefab... Then make it like that...
It really shouldn't be to hard....
If this doesn't help could you explain what you mean by "to much detail" because I've done this before and it worked fine...