I wouldn't go so far as comparing Glee to MTV. It doesn't showcase the massive amount of music that MTV did back in the day. Maybe comparing the role of MTV in the past to the role of YouTube and music sites would be more likely. I don't think there's quite the same centralisation, because you can now pick and match things to your tastes so much more, instead of having one neat place tell you what's new. However, things nowadays become popular by word of mouth, blogs, media exposure, youtube, etc.
If you're asking about the role of Glee in trend setting, then I'd have to say that it is there. I personally dislike the show as much as some of the people here do, but it's a bit ridiculous to deny that it's not important at all. It may be in decline -- although it still has a reasonably solid viewer base, it just doesn't get as much media hype -- but during its run it has influenced charts and the musical culture of its viewers. I'm talking especially about the demographics that is now in its teens, as that seems to be age group that still watches Glee and can bear the terrible story lines.

I think many people at that age wouldn't really know "Don't Stop Believing" (for example) if it hadn't been featured on Glee. At least, most teenagers I know seem to have been introduced to older music through the show, and to an extent the fact that some of these artists got some airplay, youtube views, and more sales after being featured on Glee does say something. Some more recent songs have also ended up benefitting from their Glee exposure, which works both ways: the show gets to show off how they have such varied choices of music (or whatever) and artists get some extra exposure. I'd definitely agree that Cee-Lo's "Fuck You" is an example of this phenomenon in action.