I never thought that you created this thread to just bash on Rarity. But I think that A Dog and A Pony Show was a very bad example to prove your point, the part when she whined and complained, was completely fake, that was part of her "act". But it's not secret that she can complain about things on a regular basis, it's a matter of tastes actually, and see how much you can tolerate it.
Now you say that her negative traits are "overemphasized" and affects her character, you may be right, from your point of view that is, if bringing accesories for every event or fangirling over crystals is negative to you, so be it, I can't blame you for that. But her possitive traits are there, and her potential as a character had been shown already. There's nothing else to develop or evolve. Rarity has such a complex personality that if you don't sit and watch her parts in the show carefully you wouldn't understand how deep she is as a character and how much she has to offer. You will see her as a whiny, manipulative, vain and superficial little bitch that only cares for her appearance.
again I am not blaming you, I'm just giving you my thoughts about how you used that episode as an example.