He is basically a human rainbow, and he's one of the show's biggest cash cows. Darren's always had following going into the show, what with the StarKid productions and what have you, but being featured on Glee only added to his original popularity. I'm a fan of the guy, and having met him, he's a sweetheart and absolutely deserves the success, but I can understand the feeling that he's being over saturated.
However, as someone who is a fan of the show and around many people online who are fans of the show, any number of these cast members can feel like that. Another example - Chord Overstreet, who has actually grown on me quite a bit since he was originally introduced on the show. But, Murphy did all but fire him, so I guess he's not nearly as big a deal as some make him out to be. If there's anyone on the show who is the most overexposed it is Dame Lea, who literally has a solo or two every episode she's in.
"Anybody that goes to the theater, I think we’re all misfits, so we ended up on stage or in the audience.” --- Patti LuPone.