It's pretty hard to nail down what overrated truly means, but I think the easiest way is to figure that out is just how likely it is that the music in question will sound dated in just a couple of years... Like, in the last ten years or so, the majority of what's winning Grammy awards these days is going to sound ridiculous in the next 10-20 years because of all of these dated references. For example, back in the late 90s, you had a lot of Y2K references like Will Smith's Willenium album. You'll get songs that talk about texting, YouTube, etc. How stupid would the music we enjoy from the 80s sound today if they made all these references to characters from Alf, Pam-Am Airlines, Atari video games or cassette answering machines?
Another kind of music I find "overrated" is preachy, "socially conscious" music... All of a sudden (around 1993 or so), it became acceptable for music to tell us how to think. Say what you will about the party tracks that sang things like "all I wanna do is drink and get some ass," at least they were talking in terms of, "leave me alone, this is my life, and I'm gonna have some fun." These days, we've got all these rockers talking about how this is one world we all have to live in and we all need to think about what we need to give back... That's great... For an anthropology class. But I don't want some guy with a copy of Fruity Loops and Pro-Tools pushing their personal philosophy or social ideology on me through their music.
The reason I call this overrated because for every one of me who thinks, "oh just shut the hell up" when listening to someone like Paula Cole or Sheryl Crow, there's another person who CAN identify with music like that. So that adds up to an army of people who think this music is great, not because they're brilliant pianists, composers or otherwise amazing at coming up with catchy guitar riffs... But because they write "deep" music, whatever that means. All I can think is, what's deep to you might be pure B.S. to me. Thus, overrated.