It was actually mentioned in three other posts before yours, but I digress.
South Park does fit being "overrated" for me, and
Family Guy too -- both for one main reason: it became popular and "cool" to watch them.
I never cared for
Family Guy much to begin with -- and I'm not just referring to the fact that the title character is inconsistent, abrasive, and unlikable and about as far from being a genuine "family guy" as you could get. But I don't care for it because the gags are just too formulaic and predictable for me. Every time I try watching it, whenever Peter says, "This reminds me of the time that I..." or some variant thereof, you already know what's coming: a pop culture reference or political or religious joke. That's what gets old to me.
And in my social circles, when people learn I'm an animation enthusiast, they usually ask me if I like
Family Guy and/or
South Park. That's what makes them overrated -- they've gotten so much mainstream exposure that people speak of them as if they're the only cartoons on TV for adults.
The Simpsons also fits "overrated" for me. I liked the earlier seasons, but newer episodes just don't seem as humorous as they used to be. I didn't even care much for The Simpsons Movie. It gets a lot of hype because it's been on so long, but it just doesn't live up to it.
Star Wars: Clone Wars was WAY overrated, especially in the preliminary promos. In no way has this series lived up to the expectations I had, especially after Gendy Tartakovsky's marvelous vision of Clone Wars that proceeded it. I don't hate it, but it has taken some time to grow on me because my expectations were so high. It's so hard to get past the stiff character animation, wooden faces, and cheesy dialog. I'd say it's just marginally entertaining. It tries too hard to be like the films, and falls far short.
And I would actually agree that some of the more popular vintage favorites like Transformers, GI Joe, He-Man, etc... tend to be overrated by diehard fans today. Nostalgia's rose-colored glasses obscure the shows' flaws and shortcomings. I love the original Transformers series, but as an animation enthusiast and critic it's difficult for me to ignore the continuity errors, the many animation mistakes, the poor writing, the plot holes, and the mandate from Hasbro to sell toys that took priority over everything else.
And then for diehard, purist Geewuners, the original TF series is absolute perfection. Now,
that's overrated.
And I don't agree that Spongebob is exactly overrated. Overhyped, maybe. But not overrated. It's a kids show that happens to have broad demographic appeal. I keep watching it because I've developed an attachment to the characters and the voice acting. I don't care if plot ideas are getting recycled. And I don't expect anything more or less from the show. I just watch it to be entertained.
All the other shows I've mentioned fit the definition:
http://www.merriam-webster.com/dictionary/overrated