Hmm, maybe he shaved it, and for the same reason that Krillin does: so that opponents don't have something to grab on to during a battle.
Or, judging by that pic, Nappa would be kinda old. That little kid is Vegeta, so judging by his age int he pic, Napoa would have to be in his late 30s/early 40 or something. I don't know about the hair loss and such for Saiyans, but it might be a possibility.