I suppose it's the lack of subtly or tact that ruins it.

Not even that clever. It's just, "Hey, look! Tara Strong and Ponies!! Har-Har-Har, it's topical!"

It doesn't even really work because it fails in two ways: If don't watch or keep up with MLP, you're not gonna get the actor allusion, and fans of the old show aren't going to like that they made Raven, a rather complex character, into a closet pegasister just because raisins. If you do watch MLP, however, fans who watch both of those shows aren't going to take to take too kindly to the fact that the creators had to reach that far just to make an in-joke to cater to MLP fans.

I actually think it would be more appropriate for Beast Boy to be the brony. A friend of mine actually came up with an episode idea for that, which involves Beast Boy realizing that the Twilight Sparkle expy on the show sounds a lot like Raven, and trying to make Raven say certain lines from the show or unintentionally provide voice-work for fan animations. It ends with Raven taking control of a doll of the Twilight expy, and tricking Beast Boy into stop screwing with Raven. That actually sounds a bit more clever, if I do say so myself.