Wasn’t the question put to Serino yesterday, “Do you think Zimmerman is a pathological liar?” It was after Serino said Zimm was either truthful or a pathological liar. Seems like a reasonable question to me, a non-legal trained person.
O'Mara started with an open question. What did you think of Zimmerman's reaction to being told about a video recording existing? It was Serino who said it told him that Zimmerman was either a pathological liar or telling the truth.
It was after Serino used that term that O'Mara asked if there was any other evidence that Serino took as Zimmerman being a pathological liar.
I agree, it's a perfectly sensible question.
But, if you play that same scene with dishonest cops and a dishonest prosecutor, saying that the defendant is a liar, or that a witness for the defendant is not credible, you'll come away with a different feeling about the propriety of allowing the cops to testify beyond their factual observations.