In Peanuts, a characteristic that Frieda (sometimes seen spelled Freda but it's Frieda in the strip where Linus introduces her to Charlie Brown) constantly brags about. Her first words to Charlie Brown are "How do you do, Charlie Brown? I have naturally curly hair!"

And, of course, from A Charlie Brown Christmas when the children are being assigned parts in the Christmas play:
"Do innkeepers' wives have naturally curly hair?"