Tip drill

a tip drill is what they in the rap world call giving a stripper a dollar for her to rub on you, in this case the girl in question obviously has a large butt and an ugly face so Nelly is saying that shes not good for much more than grinding on him
a woman with an ugly face but a nice body (probably big ass) who serves one man after another
most ballinest ...
He is saying that the reason people are having sex with her is because her ass is big because her face is not cute. A tip drill is that girl.