I may have an idea where “Mystery Girl” is from based on the number she gave Pearl, (301) 555-0189 (or (301) 555-0789, depending on which shot you look at).
In the real world, 301 is the area code not for Delmarva (the region Beach City is in), but for the western half of Maryland (the state which occupies most of the Delmarva Peninsula), including Maryland’s part of the DC metro area. However, 301 served as the area code for all of Maryland until 1990.
Beach City is canonically along Rehoboth Bay in Delaware, which in reality has area code 302, so what was shown is close, but not quite.
The show takes place on an alternate version of Earth, and we have seen time to time that the geography of Earth is different, both physically and politically. Therefore, Mystery Girl could be from the area that corresponds to western Maryland in our universe.
(Also of note, only 555-01xx is reserved for fictional use these days. The rest of the 555- prefixed numbers were released for public usage years ago due to the exhaustion of available phone numbers.)