Common mistake. Most dungeon tanks are new or casual tanks that don't main the role, so there has always been a prevalent belief that mitigation should be used on bosses. I had always found it more effective on trash, but I tested it one time in Sastasha and dungeon bosses did the same auto-attack damage as 2 trash enemies. If you have fought 2 trash enemies before they are nothing and the healer probably isn't even needed.
It's completely acceptable because it's story content and they are just trying to enjoy the story or level an alt. If they were trying to be good they would be doing content that requires you to be good such as extreme, savage or ultimate.the amount of people who have no idea what they are doing in high level dungeons or trials is completely unacceptable.
It is good enough. SE designed it to be good enough intentionally. They wanted to design a game where bad players would be obvious but where good players could shine and carry them. Their design was a success. This was explained in the noclip documentary (not with these exact words, but with words of the same effect).This needs to stop. We need to stop telling sprouts in mid-level content that poor play is ‘good enough’ as long as they clear.
Being expected to do anything, or to be good, comes across demanding, toxic and rude and that's what drove new players away from WoW for most of it's existence. SE understands this is rude so it can be reported.