Interesting. Is this true for all moons, or just ours? Is it shape dependent, for example are the gravity fields of Mars' potato moons even lumpier?
It appears to be the case for all solid bodies. Earth has a lumpy gravity field, as does Mars and Mercury. Not sure about Venus. The outer moons, the ones that are mostly ices and water are probably 'smoother' gravity wise as all the chunks can settle to the center easier.
The earth having a non-uniform gravity field is fun. It allows us to do thing like Molniya orbits for Russian communication satellites. It also means that geostationary satellites MUST have fuel or some way of station keeping otherwise they drift to the dips in the gravity field (one such location is the Rocky-Andes Mountains longitude) check the stability tab