Birdman exist in a universe, in which Michael Keaton can’t let go of his former days as a superhero. It’s a play on his earlier fame as the star of Tim Burton’s first two Batman movies. In Birdman he tries to recapture his earlier fame through public stunts and a Broadway show.
Michael Keaton played Batman in 1989’s Batman, and 1992’s Batman Returns. Can’t say whether those movies were better than the more recent Batman films, but they were huge in their time. Will Birdman reach the height of that superhero franchise? Probably not. This can hardly be called a mainstream film. But it will attract fans of the earlier Keaton films, along with the independent film crowd.