Are Mentors seen as unhelpful?

[Jeeqbit]

The healer was doing fine, but then when he did another giant pull, he didn't have cooldowns until when his HP was already low.

Of course, the tank should have saved their cooldowns appropriately to cover both pulls, but most tanks you meet in dungeons don't know anything or are relatively inexperienced tanking the dungeon. The last one I healed barely used mit at all except when it was pointless. The main dungeons a novice healer might struggle with if the tank doesn't use cooldowns are leveling dungeons and the first max level dungeon of each expansion, because of their item level sync.

but when it turned out the healer had gone AFK for a minute, I apologized as needed.

This is why it's not worth blurting out "advice" to strangers unless you know them outside the dungeon. You don't know if there was another factor involved like someone being AFK, lag, a mouse battery dying, being distracted, just a rare genuine mistake. And if any of those things are true, they are going to feel insulted.

However, the rest of the dungeon he was being a snark about Mentors in general being unhelpful or critical or nosy. Is this how most people see mentors?

A lot of people do, but it's not because of the mentor system. Veteran players in general are often "snarky", "rude" or "impatient" in any online game. It just so happens that a large amount of veteran players qualify for mentor status and turn on the crown.

If there wasn't a mentor system, the same thing would happen, the people behaving this way would just not have an icon to indicate they are a veteran player.

possibly because of their own experience of their NN on their home world.

The majority of people do not even ever join the Novice Network. The minority that do have a good experience in most networks because the people in there actually help and answer questions.

The negative perception seems to come from people who don't join the novice network and meet mentors in dungeons who give them "advice" which they interpret as harrasment. Because when you blurt out advice suddenly it can be interpreted as insulting and snarky unless you are very, very careful with your wording.