I'm Mormon, I went there, and there were lots of people whose behavior I didn't like. There were also a lot more whose behavior I did like.

Mormons make up 95% of the 30K student body, so if there's anyone there you don't like, they're probably a Mormon. Mormons are a big enough group (especially at BYU) that standard statistical rules apply - some are jerks, some are racist, some are angels, some and the nicest people in the world, and most are decent people with the usual character flaws. There are over 5 million Mormons in America and 12 million in the world, so they're too big to stereotype as individuals, any more than Americans, Mexicans, Catholics, Muslims, left handers, computer programmers, etc.

I'm not claiming that the single distinguishing feature of Mormons is that they're universally unkind to outsiders, but when you deal with an unusual group, it's the outliers who can really show you that there's something unusual going on. Just for example, it would completely blow my mind if I found out that there was an auto mechanic who had the same curiosity, panache, explanatory talent, etc. as Richard Feynman. This isn't because I think mechanics are all dumber than physicists, or that there aren't some smart mechanics or some dumb physicists (I know one person who I believe was a mechanic before becoming a physicist). But hearing about another Feynman-style physicist would not be such a big deal.

Similarly, the average Mormon is probably fairly indistinguishable from the average American, or the average resident of whatever country that Mormon lives in. But when you look at the outliers -- the ones with really nasty in-group hostility, the ones who work eighty hours a week and do volunteer work on weekends, the ones to whom seven kids would be a 'good start' -- you figure Mormons might be half a standard deviation more in-group centered, industrious, and fertile. And at that point, it's just Bayesian inference: you find out someone is a Mormon, you change your estimates of the chance that they have other characteristics, too.

they're too big to stereotype as individuals, any more than Americans, Mexicans, Catholics, Muslims, left handers, computer programmers, etc.

I stereotype all of these groups in the same ways. It may be misguided, but it's a good way to experience your surprises in bulk and in advance. Always efficient.