Well, I'd never heard of this Imus guy. I guess my feelings about it would vary a bit, depending on the context. Is he a sportscaster, as in reporting the game plays to his listeners? In which case, he should NEVER have made a comment like that. If he's just a DJ, esp. one who specializes in saying things for "shock value", then I can somewhat better understand why he might say that. However, I also think he should have kept in mind that there are ways to "shock" without being politically incorrect in such a high-profile, personalized way.
It is my understanding that nappy-headed a.) is usually applied only to black people, and b.) means a condition where it is obvious that the hair has not had any care in a long time; for ex. it might have accumulated lint in it. So I don't think the term would apply to someone with well-kept cornrows, or any hair that is well-styled--er, that shows that at least some care has been taken-- and not obviously dirty.