I think that you're correct about there being, for most people, an unwritten ferocity threshold that monsters must meet in order to look like they'd be powerful competitively. But then again all of the pixies are competitively viable and are small/cute, but we're just used to that and we expect it. Besides, we always know their BST because they're all the same, so there's never any mystery. Pokémon doesn't have a defined look or theme for fairies yet, and what their role will be is still unknown. Evidence for their role as support Pokémon I think can be found in the revealed abilities (Sweet Veil on Swirlix and Healer on Spritzee); this doesn't mean, however, that all fairy types will play support roles. Psychics have Espeon and Cressilia in support roles and Alakazam and Gallade as all-out attackers.
So far I am not impressed with any of the new fairy designs bar Flabébé; I personally think the fairies should look more like Celebi and Flabébé, which look more like what we're used to seeing as fairies.