Yeah, I don't put these kinds of assumptions on a par with racial/ethnic slurs or anything--I've been on the receiving end of both, and the latter are a LOT more distressing, as are of course homophobic slurs as well--but they are, at the very least, hedging into stupid-and-insulting territory, especially when said publically (and then reacted to with "Well it's true, Southerners really are like that"). Yes, I think most likely Timberlake is right about Hicks getting less support from Mississippians--and folks from his home state, Alabama, as well as what I gather is his current state, Tennessee--if he were outed (not much likelihood of that, from what I can tell from Googling bios about him), but it's tasteless to publically frame that the way he did--even if his larger point had nothing to do with homophobia, but rather the "pressures" of being an AI contestant.
Stereotyping is natural and inevitable from a sociological standpoint, but nonetheless all stereotypes are by definition inaccurate, and especially when you're talking stereotypes about people, the problem with employing them is that it implies uncritical acceptance that all individuals belonging to whichever category display whichever behavior. Get enough people freely doing that, and it will start to have adverse consequences for individuals belonging to that category, whether they've "earned" the collective rep or not.