“O'Hara's and Ashbery's innovation was to be able to pass with each other from the high to the low, to gather in their net such disparate fascinations as French Surrealist poetry, Hollwood's 'Guilty Pleasures,' Japanese Kabuki and Noh, Schoenberg's twelve-tone compositions, Leger's geometric paintings, Looney Tunes cartoons, and Samuel Beckett's spare prose.”
From Brad Gooch's biography of O'Hara. This was pretty much the origin of what we now call "postmodernism." Frank O"Hara didn't invent it, but he pretty much embodied it in its earliest form (that I know of).
It is not just an interest in popular culture. Everyone like pop culture, after all. It is the ability to shift easily and rapidly between recondite modernism and pop culture. It's not as though the difference is obliterated, either. To be a good postmodernist you have to have modernism under your belt. It's not an anti-elitist position, really. You still have to know your Beckett (or Ashbery). The emphasis usually falls on the greater acceptance of mass culture, but really it is more complicated than that.