I've never heard anyone say "uppity" in real life; only ficticious racists from the southern USA in books and films, and it's always followed by that racist word I won't write for fear of getting banned. From that I inferred that it means "getting above yourself", and has a pretty nasty meaning.
In the last year or so I've seen it used a lot on mumsnet, where people seem to use it to mean upset or agitated. I've just looked it up in a dictionary and yes, it means putting on airs, being pretentious or acting superior.
What do you understand by "uppity"?