Quote about Baltimore Being the Sleaziest city in America?

I found this in the transcript. Not quite what you were paraphrasing but the first mention of Baltimore that I could find:

Across town, located in the teeming metropolis known as downtown Baltimore live Connie and Raymond Marble, two jealous perverts that hate Divine's fame and notoriety more than anything in the whole world.

Is that it?

Whilst I have no success identifying the Specific quote you are referring to, John Walters' entire oeuvre is dedicated to re-imagining Batlimore as a sleazy, disenfracnchised neo-nightmare.

Waters many sobriquets include the 'Pope of Trash", "Prince of Puke", and "King of Sleaze", all of which he has welcomed and adopted as part of his own public persona. He also situates himself as being part of the same tradition of the artistic exploration of debauchery as Russ Meyer and Herschell Gordon Lewis.

He uses the concept as Sleaze as Sublime, and has always marketed himself alongside such imagery.

Baltimore itself has two historically predominant nicknames: 'MobTown' and 'Charm City', and according to John Berra's interpretation of Waters' work, it is the space in between these conflicting names that is being explored, and rendered.

A Quote from John Waters Autobiography, Shock Value:

Baltimoreans (Or Balti-Morons, as they are sometimes called) shouldn't hang their head in shame when they hear Baltmore being referred to as the Armpit of the Nation, or Bumberg. Be Proud! think of it as Trashtown USA, the Sleaziest city on earth, the hairdo capital of the world.

Unfortunately, and as Justin W has correctly pointed out, the dialogue doesn't seem to be part of the script from Pink Flamingos. As I've hopefully indicated, its entirely reasonable this quote (whether paraphrased or not) is included or attached to any number of his other works...

In addition to what Justin W said above, in the opening scene Divine is referred to as "the filthiest person alive." You might be mixing up / combining the two lines.