I don't know if they're terrified of Dean's popularity or feel so secure about his popularity that they know they can undermine him or poke fun at him or even humiliate him with no consequences. I think it's mostly the latter...but I do agree that they seem to knock Dean down a lot in an attempt to make Sam look better, and often wind up with the exact opposite effect. Maybe if they invested a little time in actually figuring Sam out, Dean wouldn't have to play the chump so often.