"nappy headed" is a racial epithet used by bigots. It is not in the lexicon of Hip Hop.
"Ho's" is but the two being used together belies Imus' bogus statement that he learned it from Hip Hop lyrics.

"Captain called me a 'nappy headed bastard'
"Captain called me a 'nappy headed bastard'
"Captain called me a 'nappy headed bastard'
Hurts my pride (huhhnh) hurts my pride" .......Lyric from Take This Hammer (chain gang chant)