If you were one of the 6 people on this board that listened to Mumford and Sons, then they're cool.
Now that they all listen? Not cool (the perception, anyway, not my opinion).
Of Monsters and Men are just hitting their stride. In 6-12 months, they will also be uncool.
Edward Sharpe...I don't know. I honestly have no idea what the perception is, but I have a hunch that people may have perceived him as stumbling out of the "cool" gate since they had a song in a commercial for trucks or whatever.
Not sure where Florence and the Machine sit on the spectrum, but for me, personally, they will always be solid because I like drumming on things during "The Dog Days are Over."
So in sum, like many things pop culture - it's only cool until it's well known. Then it's awful and you should be ashamed of yourself.