I used to experience the same with mentors when I was a sprout, but I don't think it's actually a problem with mentors but a problem with experienced players generally. Experienced players are just more likely to have achieved mentor status from having played a lot.

They have seen these dungeons before, so they get impatient in them and want to get through them quickly and that can cause them to leave or express frustration when it doesn't go as well as a previous run they did.

The mentors who participate in the novice network answer questions all of the time and help people queue into content faster for the first time, but the majority of them probably don't participate in the novice network. It's fine to not be in the novice network but they should only wear the crown if they actually want to help and be polite and considerate of the fact that someone in a dungeon is new or returning.