too much stereotyped “urban” in my urban fantasy

Earlier (wayyyy earlier) in this thread, I posted my disappointment in the Jim Butcher Storm Front novel, and someone posited that urban fiction is a stronghold of racist chauvinists. I thought that was a bit strongly worded, but now I'm reading Larry Correia's Monster Hunter Vengeance, which was a $1 download from Audible. He's using a mixed racial background as his protagonist, so I've been cutting him some slack, but between the strong gun-nut/anti-government/libertarian tones and now a pack of "urban" garden gnomes who ape black culture as comic relief, I'm finally seeing the pattern.

Is this really that common a theme?

How is it that it carries through the genre?