Just found InfoDump#8, interesting reading.
Following the various links, I see Momo actually has articulation seams at the shoulders (seen when she has a shower and is walking around in a towel). The neck seam comes and goes, so it seems fair to infer she has seams at the hips as well. Hanners plays with her squeaky skin, and Claire's brother remarks on her skin as well. InfoDump #8 doesn't answer the question, but implies fairly clearly that she covers herself because she perceives it as a "human" thing to do.
I've seen suggestions she is "anatomically correct" but I can't see anywhere that actually states this, it seems to be an extrapolation of the "live eels" throwaway joke when she first appeared.
So it seems as though she would look much like one of those Japanese animated figures?