I'm not using the film's popularity as a merit of the film, but rather a marker of it's merit. And the comparisons you use are simply inapplicable. While many people may like Miley Cyrus or Dane Cook, many other people have heard their material and don't like them. This isn't true with the Dark Knight; the only people I've heard talking in any negative fashion are all contained on this board (and that's why I bring up the IMDB and Critic ratings - they indicate the overwhelmingly positive reaction to the film).
Let's divert to another example: No Country For Old Men. Most people who saw the film in theaters, I think it's safe to say, tend towards the more sophisticated, "artsy" side (they swayed towards what we might call cineastes - that is, not cinematic laymen). Of those who saw the film, the vast majority enjoyed it and deemed it a good movie (as documented by the critical response and IMDB score, for example).
Now shift back and consider the scope of The Dark Knight's audience. Here's a film that pulls in EVERYONE; for various reasons (including the blockbuster appeal, the film-buff favorite Christopher Nolan, and the Heath Ledger tragedy) The Dark Knight has been able to pull in everyone from the hardened Fellini and Godard loving cineaste to the I-Wont-Watch-It-Unless-Directed-By-Michael-Bay chump. And the reaction has been darn-near universal. All this doesn't even take into account the DEGREE of positive reaction; while Dane Cook might be popular (and I agree that he really shouldn't be), he doesn't generate the kind of fervor, even passion, that The Dark Knight does.
I will gladly examine the merits of the film itself in it's defence - hell, I'll probably end up writing a few papers on the film some time down the road. But my point here is this: to simply jump online and call The Dark Knight a bad film and say "popularity is irrelevant" is ridiculous. A film simply does not produce this level of buzz, excitement, momentum, energy, and interest without a substantial degree of underlying quality driving it all. You don't have to like the film, but I do think you have to acknowledge how important it has quickly become.