Interesting question when put on this site. I sometimes wonder how folks put up with Frontpage slopimation.
There's a lot of conjecture about why ugliness is chosen for promotion. One would typically think studios are starved for talent, wherein the best work could be reproduced. It's like ugly is the lowest common denominator for the team of animators. Ugly is easy to remake. Maybe ugly is fast and easy, to cover for a better project's failures in scheduled releases.
What if these publishers want to starve the masses for the good shows? That's against the company's viewer-and-monetary interest, unless they're a captured institution by some other interest. Maybe there's an executive decision to make some shows like shit so others appear better. Content curation has been noted on youtube, for example, where most everyone agrees the popular stuff is obnoxious, yet it stays popular.
Maybe these shows are indicative that audiences don't know what they want, or that creators misinterpret what audiences want. What if the proliferation of ugly in shows like Problem Solverz is indicative of deliberate, closely measured quantities of what audiences are capable of tolerating? Maybe the studios want to know just how sloppy the quality can get before audiences leave? Adult Swim doesn't care much with their limited night-time viewership, and their quality is shit as demonstrated by a show like squidbillies. Maybe adults are more willing to tolerate ugliness for the expectation that the show is a comedy. Maybe these cartoons are looking to increase the audience's tolerances for shit, so producers can be more lazy yet still get the same viewership for their ad-twats. What if advertisements were more pretty than the shows?