TV
If you can’t say anything nice, say it about Diane.
— Carla • Cheers

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
A snappy twist on the familiar advice to stay silent if you can't be nice: instead of silence, the speaker suggests saying it about Diane. The joke works as a jab, implying Diane is the one person it's fine to speak unkindly of.
Context reviewed