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Similar to your mom jokes, but making fun of how hipsters reject anything as soon as it becomes popular with the masses, or better said, mainstream.

Basically you call one thing mainstream and go on to praise another thing for some kind of (retarded or otherwise) reason.
Examples of ...is so mainstream jokes:

Lady Antebellum is so mainstream, I prefer Wilco because Seth Rogen mentioned them in Funny People.

Pokémon is so mainstream, I prefer Monster Rancher because it never got its well-deserved limelight.

Francis Ford Coppola is so mainstream, I prefer Stanley Kubrick because his films make you think with your brain instead of just see with your eyes.

Sea otters are so mainstream, I prefer river otters because they don't pollute the ocean with their trash and feces.
by JimboWales May 10, 2011