Perhaps I don't know enough Mormons?
Book of Mormon - no matter how many Mormons you meet, your experience will always be different than harmony's, and I'll warrant Mormons will always look - and BE - nicer to you than her.
The niceness of Mormons - aside from personality variations which can be found everywhere - is sometimes manipulative. Mormons are taught to show their best side to the world, to show how HAPPY they are, so that the rest of the world will say: WOW! Look at how happy that person is. I wonder what their secret is. I wish *I* could be that happy.
And that is supposed to be the opening to missionary work.
Another aspect of Mormon niceness is the fear of being "bad". Mormonism, while it does accept Christ's atonement as a free gift
in terms of everyone resurrecting with a body and in terms of opening the door of opportunity to exaltation, in the end, Mormonism is a very works-based religion. You have to earn your way to heaven (or the CK). Moreover, so much is asked of Mormons that it is very easy to feel inadequate. So being "nice" and happy is almost a religious obligation, and if one isn't "nice", one might not get into the CK.
Then there is the guilt factor - I experienced this a lot - Mormons are taught, over and over, that they are the LUCKIEST and most BLESSED people on the face of the entire EARTH and even in the history of the PLANET (due to the great apostasy) because they actually have the ONE TRUE GOSPEL OF JC - so Mormons tend to feel guilty if they don't feel happy, nice, shiny.
But this forced happiness and niceness sometimes covers depression and anger, which can be manifest through passive aggressiveness (always being late to meetings, over eating junk). And, with certain people, this aggression can be openly manifest - particularly towards apostates.
Moreover, the "niceness" overlay aside, some Mormon teachings are quite cruel - swedeboy mentioned the homosexuality issue, which is one example. There are other potential cruelties - such as racial attitudes or past teachings, the tendency to "idealize", and hence, control women, the association of righteousness with financial success, etc.
I'm all for basic politeness, but when nicety is a veneer coating, it is less appealing.
Having said that, please don't interpret this to mean I find all Mormons unpleasant. That is not true at all. They tend to be like the regular population in that regard. Some pleasant, some not. They just tend to cover up the "not" part more than nonmormons (although I think other religious sects have the same tendency).