Basically, Popcorn munchers are what Hollywood calls The Lowest Common Denominator they're the kinds of people who will ensure the latest Michael Bay explosion-fest is number one at the box office. They're the kinds of people who will tell you that the recent re-make of a classic film is better because it's more action packed than the original which is boring because it has "Too much talking".
Brian: "I hear people don't like the twist at the end, and all that spiritual stuff"
Roger: "What? I thought that was so clever! I never saw it coming."
Brian: "Yeah, so does everyone else I've talked too. I have a feeling the people who hated it were just a bunch of Popcorn Munchers who went to it thinking it would be more action packed"