The Rock object is not good at creating smooth surfaces, but it wasn't really designed to. The Rock object generates angular shapes, and it does this using boolean intersection of a relatively small number of planes (e.g. 20). From this base shape you can subdivide it (on the Mesh tab) to create a detailed triangle mesh which can be deformed with displacement shaders. The algorithm has O(N^2) complexity so it may be slow if you try to use it with thousands of faces, especially if you also generate a large number of "unique variations".
If your goal is to converge on a smooth shape, it's better to use an imported sphere, or the Poly Sphere object which will be released in an update this year. Imported spheres can be deformed using mesh deformation just like the Rock object can (in the Professional edition). However, one of the advantages of the Rock object over an imported model is that it can generate "unique variations" which are useful in a population. I may be able to add that capability to the Poly Sphere.