The big issue isn't that she's white, there have been white rappers before her.
(Isn't the best selling rapper of all time a white man?)
The issue is that she is essentially profiting off of a caricature of the stereotypical black woman: the cartoonish booty, the phony accent, it's all too much. It's almost like reverse blackface in a way. The fact that she also makes wack, cornball music that is hawked as real rap makes it even worse. Having someone like Iggy or Macklemore being praised and winning all types of awards for their bubblegum fluff while deep, soul-stirring urban music is swept under the rug by mainstream media is a problem.