Riff Raff invites hate for a few reasons. Arguably, he's perpetrating a stereotype, and playing up a one-dimensional character. There are plenty of good reasons to suspect that the appreciation of his art has a strange relationship with race. One needs only to look at the relative lower profile of former comical partner-in-crime and African-American performer TKO Capone; how did Riff Raff leap frog ahead of him so suddenly?
There are also some less dramatic reasons for the hate he receives. Riff Raff is a performer more interested in entertainment than creating serious art, and for a certain strain of hip-hop fan, there's nothing worse than a rapper whose goal doesn't involve dropping a new Illmatic or Ridin' Dirty. As a rapper, the Caucasian sensation has had his fair share of catchy, funny songs, and it's understandable why people are drawn to his outlandish persona.
But he also provides quite a dilemma; similarly amusing Texan drawlers haven't ascended to the same profile, and it's not difficult to see that there might be a level of identification going on with his audience (down to the high-profile Diplo cosign) that gives him an extra boost. Riff Raff isn't taken at his word; it's all a joke, while non-white rappers are presumed to lack that self-awareness.