This bit, (Below), I understand fully. If the comment is about belittling someone, then it is out of order.
Having seen some of the folk that FA/CC put up with, that bit I understand.
What I do object to is the idea that prevails on some flights that the customers are only there to get in the way of CC doing their job. That attitude can be easily caused by poor leadership.
"The problem is, when used in the context of a commercial airline flight, such a comment by one individual passenger to one individual crew member, is invariably intended to belittle that crew member. And in that context, it is also incorrect."