As Splott Man said, "natty" in England means smart. That might be what Bob Marley meant - or perhaps it might have been short for natural.

I'm a bit taken aback at the idea that "nappy" as a word for hair should be considered demeaning. I've always assumed it's just a way of saying that hair is short and springy. I suppose it's a reflection of the fact that in a prejudice-affected culture any descriptive word can take on an insulting meaning.