Bike Chick, my head and hair are totally sweated up (that's not a word, according to my computer), always. It doesn't matter what the temperature is. Wearing a buff, cap, etc. doesn't really help at all. It might make my hair a bit drier, but I really only wear a head cover for warmth in the cold.
I have seen those women who look totally dry, with smooth, shiny, straight hair after doing a hard ride. I guess they just don't sweat the way I do. I have one kid who rarely sweats and one who is like me, so I guess there's a general variability in this trait.