I find that most "art house" films are hopelessly mainstream. I honestly hear more about films by Darren Aronofsky on both the internet and in real life than anything else. Most of these films are, at heart, just as much pointless visual orgasms as the critically panned big budget action/science fiction movies with 2,000 plus visual effects shots and pyrotechnics. I don't see this as degenerative or progressive. Just entertaining, and perhaps, in some ways, inspirational to the viewers. For example, if it weren't for films like Star Wars, Jurassic Park, or Indiana Jones, I would've had no interest in filmmaking. I would hardly consider that degenerative.
I find that movies that revolve around people over special effects are far more degenerative, case in point the Twilight movies, which have an amazing ability to brainwash its target audience into making them think they have to be a certain way. But that doesn't mean they're all like that. And more likely, it's just the impressionable target audience rather than the movie itself.
But to answer the question, does mainstream = bad? No. Mainstream = neutral, and it all depends on what you do with it and how you view it. But I think it's absolutely pretentious and absurd to think that anything popular is the devil.
PS: I'm sorry if my writing isn't concise enough, I'm sorta drunk. And I might consider expanding my thoughts later.