Render -> Solid Clouds
Set to "turbulent"
let it render
invert
adjust levels as appropriate.
If you want only parts of the height map to have mountains, make another solid cloud layer, adjust its levels, and use it as alpha mask for the actual mountainous height map.
You should end up with something like this:
Which as such doesn't look too impressive, but when you use it as a height map (here simply embossed to show the effect):
(watch out for tresher maws, they nest on flat areas)
This can then be overlaid on top of ground texture, which can also be made with gradients from the height map:
Of course, in a 3D-program you would just use that as a height map (or make a normal map out of it), but making stuff in 2D is also quite interesting.