There is no requirement for moons to be spherical. See Saturn's smaller moons, including Hyperion.

I said something similar over at /r/asteroid about a year ago, about them being captured asteroids, and got refuted by a post-doc who knows more about the Martian moons than I. Now I'm not totally convinced, either way.

Personally, I think we will know a lot more after the JAXA probe brings back samples.