Well, as long as you've got the gun, the glasses/goggles and probably the hair, I think ANY amount of street clothes would be easily accepted. The trick would be less on what you wear, and more how you present the character. If she has a catchphrase/standard pose/whatnot, let yourself rely on it and BE the character while you're out at the con.
But if you were really trying for the street clothes look, I'd search out a yellow sundress, with maybe a white t-shirt underneath it. Or if not a t-shirt....I'm not sure what kind of clothes it's called, but I know it's not a bodice. Top it off with some comfortable general purpose shoes (not a sneaker, or anything really logo-ridden). IF you were shopping around and found some old riding boots in your size, you could just draw on the red laces.