Mark Mayerson links to that King of the Hill pitch reel, and he and his readers correctly point out that the artwork on that show is not very pleasant to look at. Or, for that matter, to draw.
The thing is, though, that I don't think King of the Hill would be better if it were better-drawn. The look of the show is kind of ugly because that's the way Mike Judge draws. But I don't think there is any way the show would have been as good or as successful as it is if Judge had hired somebody else to design the show for him.
One of the reasons a TV show depends on a strong showrunner is that television episodes, which depend on a large committee of people to write, direct and produce them, need to have a guiding hand, a set style that makes every episode feel like it is part of the same overall "voice." (Even if episode 1 and episode 2 have different writers, the characters need to be written similarly in each episode.) In an animated show, the need to keep everyone on the same page is even greater because you've got complete separation between several different components: On The Simpsons or King of the Hill, you've got the writers at Fox, the U.S. artists at Film Roman, and the actual finished animation is done in Korea. If the writing and the animation seem to clash with each other -- if the show doesn't look the way it's written -- then there's no way that an animated show can achieve the organic feel, the one voice, that it needs.
You see how this applies to King of the Hill: Mike Judge doesn't draw pretty, but he draws the way he writes. The concept of the show started not with a script but with a sketch, of the four guys in the alley saying "Yep." That's organic: the writing and the drawing work together to create the joke. And while Hank Hill isn't the best-drawn of characters, his design fits with Mike Judge's voice and Mike Judge's writing ideas.
If you look at the prime-time animated shows that have succeeded, they all involve a writing and drawing style that meshes well, even if the drawings aren't pleasant to look at. Lord knows I don't like Family Guy, but Seth MacFarlane is another guy who draws the way he writes: the show looks like it sounds, and everything fits together to produce an organic piece of crap (what could be more organic than that, come to think of it?).
The prime-time animated shows that fail, on the other hand, are often those where the design style is not created by the person or people who came up with the show. That's not the only reason they fail, of course, but it doesn't help. Look at The Critic. Al Jean and Mike Reiss, the very funny and talented Simpsons producers, created the show and the characters; not being cartoonists, they left it to the very funny and talented Simpsons director Rich Moore to design the show. But though the creators unquestionably had input into how the show would look, there was always a certain non-specificity to the design style (you didn't really know why the characters should look this way) and maybe even a certain tension between the lush, colorful backgrounds and the rapid-fire cutaways and movie parodies the writers were coming up with. The show was, by most standards, better-drawn than King of the Hill but the drawing just wasn't inseparable from the writing. On King of the Hill, it is, and rightly or wrongly, that's more important than the drawing actually being good.
One more thing to add is that the characters on King of the Hill are less ugly now than they were when the show started; sometime in the second season, Wes Archer, the supervising director (whose short "Jac Mac and Rad Boy Go!" was a big influence on Beavis and Butt-Head) re-designed all the characters to simplify them and make them easier to draw and look at. Season 2 Hank looks better than season 1 Hank, who in turn looks better than his inspiration, Mr. Anderson on Beavis and Butt-Head.