I think ... not really usable for a girl. On a girl especially it reminds me of muppet and moppet and muffin and little miss muffet, and Moffett Field (idiosyncratic - I grew up near there). Moffie basically IS Muffy, isn't it? It's so close. I also see "mo fat" in it (although I probably would not quickly notice that IRL).
Only because it IS surnamey, it would be passable on a boy. NMS though.