It could mean that they shop a lot, or, do something other than shopping. People who work at malls aren't considered mall regulars since they go to the mall to work for their living instead of randomly going.
Andrea: Again? But we went yesterday..and haven't you been going to the mall every weekend for several months now!?
Tiffany: Yeah..so what?
Andrea: -sigh- You're a mall regular. I bet every shopkeeper in the mall knows you as well as if you were their blood-sibling.