As Newton showed, a sphere, gravitationally, can be considered to have all of its mass located at its center. The equal distribution of mass into concentric shells allows for this mathematical assignment of a point source to a sphere.
While the mathematics works within acceptable parameters effectively for larger masses, such as stars, it does not effectively replicate the situation when the mass involved is that on the atomic scale. Any external irregularities on the surface of the star for example, would have virtually no impact upon the calculations of gravitational attractions.

However, the nucleus is not a sphere, but an agglomeration of spheres. That means that the external shape of the nucleus could in fact, only possibly be absolutely spherical when it is composed of, by a single proton. With the agglomeration of other nucleons, the ability for the nucleus to remain a sphere, is of course, impossible.