Lyric: "Now tell me that ain't insecurr/The concept of school seems so secure/ Sophmore, three yurrs"
In scholar bell hooks' seminal essay "Eating the Other: Desire and Resistance," she examines how, when white men enter into the cultural space of another ethnicity, the idea becomes appealing because primitive fantasies arise. So the notion that by stressing urr, Kanye is trying to "mock some bourgeois black peoples' fetish with ghetto pronunciation of words" is untrue.
That RG believes bourgeois black people fetishize about pronouncing words with a certain accent is also pretty absurd. This is what hooks might interpret as the "consumption of otherness," whereupon whites place narrow-minded restrictions on "the other" through the occupation of a particular cultural space. In truth, Kanye probably just liked the sound of "insecurr" and felt the need to stress the pronunciation in the subsequent lines to pull off his playful rhyme scheme.