I don't understand all the animosity toward Diamond Tiara. She can illmannered at times, but so are all the kids in town. She's caused far less trouble than the CMC, and never once doomed Ponyville with certain destruction like Snips and Snails, and yet she's hated and they are not. What's worse, messing with Apple Bloom or causing a giant monster to rampage through downtown? It seems the vote has been cast for the former. She even praised the CMC multiple times in Ponyville Confidential for all the dirt they dug up for the newspaper, so it seems like she has nothing personal against any of them. It may very well be (and I'm taking a shot in the dark here) that her teasing is an attempt at pushing Apple Bloom to find her talent. After all, diamonds are formed under great pressure.
While there is certainly room for refinement and polishing, that's what growing up is. It's something everyone has to go through. No one (or at least very few people) is born a perfectly amiable member of society, it's something we're supposed to learn while growing up. Civilisation is not innate, it is engineered. Diamond Tiara is no worse than any normal human child. She has great drive and enthusiasm, she just needs some direction and then I expect great things for her future.
