Playing poorly is not acceptable. The reason you think it is acceptable is because not many people are complaining about it in game.

We are not allowed to discuss dps numbers (ACT is not allowed to be used that way) so you can't tell people that they are doing low dps. Not using proper rotation, buffs or AoE, all that can be noticed without the need of ACT. So you can give people advice on how to get better.

When people receive an advice, most of them act like victims. Respond back rudely or ignore you. It can be that the advice was not delivered properly or the person receiving it is just too sensitive. Either way, many people just stopped bothering with it.

Words like "pro parse", "elitist", "toxic" gets thrown easily at anyone giving advice. It is not worth getting called bad names because you gave an advice in a brain dead content.