I don’t know what mumble rap is, so I’m not commenting on that in particular.

But, something being selected for doesn’t imply that it isn’t a bad thing. Even if we assume that “how much the typical person likes it” or “how many people like to listen to it” or something along those lines is a good measure of goodness, those are not the only things involved in the selection pressure. Things like cost to produce, discoverability, etc. are all also things that influence the selection pressures. And if there are contributions to the pressures which aren’t entirely aligned with the direction of good, then it seems entirely possible that the selection could make something worse (as in, not as good, not as in, less successful)