Actually, it does not make one a singer, period.Sorry to burst your bubble Hip Hop fans but being able to rhyme,using heavy amounts of bass, adding some popular song looping in the background, and using Autotune does not make a good singer.
That's the underlying "grand hubris" of Rap. It's performed by people that believe that what they have to say, is too important to bother with trying to learn to sing. If they discovered they didn't have the talent for it, that would deprive the world of their "monumental revelations", and "gravitas opinions"! we couldn't have that now could we?
Yeah we could, and for that matter, should.