"Ignorant" is an over-used descriptor of rap music. In a commonly expressed extreme, it could describe party rap, misogynist rap (although not all misogynist rap is considered "ignorant," inexplicably), violent rap, rap that isn't concerned with lyricism, rap from certain regions, and rap made by people without formal educations. Many times, it's a catch-all term used to describe music that the critic simply doesn't understand.
But there's a certain constellation of rap music that, when fetishized by white people, causes arched eyebrows across the board. When a white person's interest in a hip-hop artist seems driven almost entirely by how hood they are, how "real" they seem, it suggests they may, in fact, have a less-than-real relationship with black people, in general.
White people aren't alone in fetishizing authenticity, but when they do it, it's much more likely to feel racist, like they have a fantastical idea of how black people live that's driven more by caricature than truth.