I can't tell,But I'd always thought she'd be pretty with long brown hair!
BTW,They never did show her face at all but when time when there was this one episode where the muppet babies about being a monster or something like that before and what happened was nanny imagined her self being a monster and it did show her face before which was just a monster with long pretty brown hair or so.
na! she wouldn't have been blound hair! and I'm assuming since the moster had brown hair, so would she.. cource, for myself, I always assumed she would have short brown hair, and probably wear a hat most of the time