It's an interesting point, I think, and it has at least a grain of truth in it. A lot of people are rather willing to change their views and jump on the bandwagon as soon as whoever's carrying the wagon acknowledges that the wagon exists (or something, that's rather a weak way of phrasing it). BUT, I think it's jumping from one bandwagon to another. The people I see saying this like "I usually don't like Mumford and Sons, but..." are the same ones who previously have ragged on them so much that it's obviously over the top, obviously not their own, rationalised and thought-out opinion.
I have never knowingly heard a Mumford and Sons song. If I heard them, I wouldn't know it. As far as I know, this was my first exposure to them. I was pretty surprised by how "big" they are.
You've probably heard them by now -- particularly since the guitar in the first minutes of "The Cave" sounds totally like a song on The New Green's album. Which one? I'll get back to you on this.
Hmmm... I've listened to this song all the way through and I can't guess which "me" song it sounds like. Do tell :)
I first came across them probably as they hit the first sort of "big": when "hip teens" start talking about them and playing covers at open mic nights. Because of that, they've always had the association in my mind of folk catering to the popular music crowd, as ButterflyEffect described. They've gotten bigger since, to no surprise, and, also not surprisingly, most of those "hip teens" who were talking about them and playing them a few years ago are now the people who hate on them endlessly.