As an LA native, I feel it my duty to weigh in on this. The term "vally girl" refers not just to *any* stupid Californian, but to one fro mthe San Fernando valley, a particularly unpleasant suburb of Los Angeles that must have one of the highest strip-mall-to-person densities worldwide.
It's a place that bakes in the summer and freezes in the winter, the kind of place that, with a terrible climate, less culture even than most of Los Angeles, and hiddeous civic style, makes living in LA utterly pointless.
The Statman Thesis conjectures that the place itself -- the San Fernando vally -- is a veritable breeding ground for all manner of annoying habits. Whether traceable to its marked ability to attract residents with absolutely no taste or the lack of anything better for them to do than think up no ways to be irritating has yet to be discerned, and begs for more ample research.