I've always looked at it like this. Say we lose tonight.....
Saying things along the lines of: Our guys suck, Javin is a waste of a scholarship, Moore is terrible and a walking turnover, I wish Jack never shot a three again, etc........These are all examples of stupid things to say that should pis people off.
Constructed criticism IMO is always cool. "I wish Hurt spent more time near the basket and less time 30 feet from the rim." "I love Tre but he needs to take better care of the ball." "I wish Wendell dribbled a lot less."
Challenging K should be fine too as long as it's in context. For example, I will go to my grave thinking that K failed last years team. He failed to install an offense, he failed to keep our guys motivated, etc.