As she was leaving, Diana noticed Peter's sweater (seen here as Peter greeted his mother-in-law). "Goodbye, Dr. Huxtable," she cracked.
Going strictly on the intent of the scriptwriter, it was supposed to be a reference to 'The Cosby Show', in which Bill Cosby played Dr. Cliff Huxtable (who was known for his sweaters). Thankfully, Channing Powell trusted the audience to get the reference without hitting them over the head with adding a mention of the show's title.
So there's no Zonk on that level; Dr. Huxtable was being mentioned as a real person in the same world as FBI agents Berrigan and Burke.
But why would Diana have known him, and have said his name as though she knew Peter would understand who he was?
In ordinary circumstances, I might have said that Dr. Huxtable had done something to gain fame, perhaps even notoriety, in the years since the show went off the air. Instead it's on a more personal level. And I think that answer lies with Peter and not with Diana.
I believe Peter and Elizabeth live in the same general neighborhood as the Huxtable Family.
At some point, either in the past before the series started or off-screen between episodes, the Burkes may have thrown a neighborhood block party and invited the Huxtables over. And perhaps co-workers like Diana and Agent Jones were also invited.
I'd like to keep the idea running and claim that Diana may have even dated Dr. Huxtable's daughter oldest daughter while Denise was in an experimentation stage. But I don't want to push it - that brings me deeper into fanfic territory and that's your job.