During the run of 'The Addams Family' (the original series), most people didn't know who Gomez and Morticia and the other family members were when they first came to call upon them. But you can be sure that nobody ever forgot them afterwards!
The show ran only two years but the lives of 'The Addams Family', as with all TV characters, continued in Toobworld. And at some point during that time, their lives caught the interest of Hollywood - perhaps it was the time when Gomez suffered amnesia and then traveled to Gotham City, where he took on the persona of The Riddler, one of the foes of 'Batman'.
At any rate, after many years in development, a theatrical movie was filmed about "The Addams Family", both in Toobworld as well as in the Trueniverse. And in both dimensions that movie was a very big hit worldwide.
So when Billy Gilmartin referred to his daughter Grace as "Morticia" in the first episode of 'Murder City', it was because her Goth make-up and wardrobe reminded him of the character he had seen in the movie, who was based on a person just as real as he was in Toobworld; and not to a character from a TV show.
No Zonk here!