I get the impression that cartoon styles get uglier and uglier. You don't get anything near the quality of old Disney or Looney Tunes cartoons. Modern cartoons look just nasty.
I think there was a Family Guy episode where they actually get into a Disney style universe. And all I could think of was how much better the drawings looked.
Yes, it does seem like the newer animation isn't as visually appealing. But then again, Futurama (aside from the previously mentioned icky Zoidberg!) is very lovely to look at.
I think The Simpson's does a great job of portraying unattractive things is a way that isn't visually repulsive. For example, you get the idea that Homer or Moe are not attractive guys, but the animation itself is still nice, doesn't make you want to look away.