Overrated:
My Little Pony: Friendship is Magic (at least from an objective point of view; I still find the show to be charming in some weird way despite its average storytelling and wasted potential-read the MLP thread for more criticism!-so I guess that's saying something.)
Ren & Stimpy (I don't know why, I'm just not too big on this show. I like how absolutely insane the animation looks, and some episodes can be quite funny. It's probably more clever than the many other gross-out shows it spawned, but I don't remember it nearly as fondly as Courage or Billy & Mandy. The fact that I was born the year after the show was cancelled might be a key reason here.)
Rugrats (I used to kind of like it as a kid, now I just think it's sort of boring. Then again, I haven't seen this one in a long time.)
Phineas & Ferb (see Rugrats, although I do like how the show subverts cliches in its humor.)
Regular Show (once again, probably overrated in an objective sense as it follows a pretty basic formula: Character has a mundane problem, character finds or does something weird to try to solve the problem, the mundane problem becomes a weird problem, the characters have to band together to solve the weird problem, throw in some fighting or a montage with some '80's song in it, and you have pretty much most of the episodes of this show. Also, as much as I like Mordecai and Rigby, I wouldn't really call them good role models. Some of the stories/endings are pretty half-baked, too. Still, it's funny, stupid, and nostalgic enough to keep my interest.)
Adventure Time (while I do respect the show actually taking advantage of the large world it's given, developing the locations as well as the characters and actually managing to hold on to several story arcs that actually change the status quo of the series, I just don't find it as funny or relatable as Regular Show.)
Littlest Pet Shop (I've heard that a fandom has grown for the reboot of this show, too, based on the bronies' influence. Sorry, guys, but one show based on a toy franchise for little girls is enough, thank you very much.)
Pokemon/Digimon/Yu-Gi-Oh/Beyblade/etc. (I have absolutely no idea why any of these shows are so popular since the same thing happens in every single episode and most of them are based on the same concept--battling and collecting monsters to fight with other monsters. I know it's probably a sub-genre of anime and what I'm saying may not even be accurate, but I still don't plan on watching these shows anytime soon.)
Underrated:
Camp Lazlo (It's made by Rocko's creator, Joe Murray, and it has a simple plot with okay enough characters and humor and even some memorable stories. Remember when the Squirrel Scouts actually thought the Beans were aliens or how Lazlo made his own secret club with his imagination? Nothing amazing, but still a fun little show.)
Chowder (Sort of in the same boat as Camp Lazlo--it has a writer from another well-known cartoon (C.H. Greenblatt from SpongeBob) with simple but memorable stories, and good enough characters and humor.)
Homestar Runner (I don't care if this isn't a TV show. I don't care if its Internet relevancy died out years ago. Strong Bad and his emails might be overrated themselves, but the entire website's library of cartoons is still hilarious. Please watch it.)
Home Movies (crappy designs and animation, but it has funny and relatable characters and stories. The music is cool, and Coach McGuirk is a riot. Also, like the last cartoon I mentioned, this one featured They Might Be Giants as guest stars, and you all know how I feel about them... :P)
Dan Vs. (I seriously don't know why more people don't watch this show. The main character is a jerk, but actually sympathetic and pretty funny; then again, I do have a preference for characters with snark. The rest of the characters are good, too, and Elise is actually kind of kick-butt. The show follows a formula, though not one as confining as Regular Show's, and actually leaves room for developing comedic situations based on getting revenge for things. Its attempts at satire, such as on "Dan vs. Technology", aren't bad, either. The only criticisms are that the animation is pretty lacking at times, and it plays out more like a sitcom than a cartoon. Some episodes are definitely weaker than others in the humor department. Still a show you should check out.)