It's a girl who comes from a more affluent, more educated part of a city, or belongs to an upper social class of people.
The noun "uptown" does not always convey this meaning; it can sometimes just refer to the northern part of a city. But as an adjective, uptown is used to convey a sense of upper social class. It can refer to an object or to a person.
"That new dress gives you a very uptown appearance!" (You appear to be rich when you wear that dress!)
"The new bus station is located uptown." (Location only)
EDIT: You had it covered, fdmaxey! Didn't see your answer before I posted.