Wesley Willis has had an intense, broad impact on both hip hop & spoken word; I just read some articles about him earlier this year. The Beastie Boys are big fans, as are the Chili Peppers, Henry Rollins, etc.
I haven’t heard any of his work but from what I’ve read, he reminds me a lot of Daniel Johnston; mentality-wise, art-wise, etc.
I find that the so-called mentally ill are frequently tremendous artists. But unfortunately there are people who disregard their capabilities & treat them like sideshow freaks, and who go to shows expecting only the insanity and not the genius. You get what you give, I guess.