As true as your observation may be (and I would agree with it), it is still possible to dislike Romney while being neutral or even supportive of Mormonism. I have a couple of Mormon friends who dislike Romney (one of them intensely). They obviously don't dislike him because of his Mormonism.
But Mitt is also everything that McCain is not.
If Mitt Romney conveys in attitude what you just wrote, it is no wonder he is disliked for being arrogant. Class envy has two ugly components: the "have nots" being jeolous of the "haves" and the "haves" being contemptuous of the "have nots." Given the widely reported dislike of all the Republican candidates towards Romney, it is very possible that the second component is the reality rather than the first.
I think there was a lot of jealousy because Romney is smart, dazzingly successful, squeaky clean with a squeaky clean family, and handsome. But the most obvious dislike came from McCain and Huckabee. Huckabee’s reasons are obvious. He was whipping up the anti-Mormons.
I’ve never met Romney, but I’ve never seen him be arrogant. His attitude has always been gracious, even when he was under attack.