Maybe Pinkie is one of Discord's descendants or something, though I once had the idea for a fanfic that Pinkie was 'Mother Earth' manifested as one of her 'children' and that was why she has so many oddball abilities (iow, she's secretly a Goddess).
Ah, see, that's where you're wrong. Chaos and ORDER are complete opposites. Harmony is not order. Harmony is balance. Specifically, a balance between order and chaos. Complete chaos and complete order both would create an uninhabitable universe, thus we need a happy medium in order to survive. That's what harmony is all about.
Laughter represents the chaotic element of harmony, and how chaos can, if used in the right way at the right times, do good in the world.