When I still had my sub running (I decided to let it lapse because I'm pissed that someone who's been harassing me indirectly through defamation hasn't gotten any punishment at all despite doing that stuff for years to many different people) I would try to offer help on NN as a mentor, though as of late I've been doing it via a sprout alt because i ran out of things to do on my main (mentor) character.
Generally, and while the specific atmosphere differs from server to server and by time of day, NN is a lovely mess due to its nature as an open forum. I can deal with it most of the time, but sometimes I need to respond in /tells for one reason or another. There's a lot of people in there, from people who want to help but don't really know that much, to people who just want people to give them attention, to people who deliberately stir the pot, to people who mean well but have issues with phrasing/etc, to people who feel entitled to people's time and attention, to people who like listening to other people, to people who are dense as hell, to people who understand things fast, to people who get vendettas and act on them in public, to people who like helping but get drowned out easily / are dainty.
I'm definitly of the 'likes to help, but is unassuming/dainty' group, hence my relying on /tells sometimes--- i get drowned out otherwise.
My worst experiences were with three kinds of people: First, sprouts who felt like they are entitled to us giving them their undivided attention -- i would read their question and try to remember the answer, but then they'd start complaining and insulting because 'nobody responded' and i'd just delete my response because that is SUCH a big turn-off. Second, returners who think they know better than everyone else, can't accept corrections/new information, and keep parroting years-old advice like 'just shirk the healer and cover them to deal with tankbusters'. Third, people -returners, sprouts, and mentors all- who treat the channel as a joke and use the "NN bad" meme as an excuse to troll.
Also in general, you can tell early on which sprouts would become problematic mentors-- many of the people i have issues with I had pegged as being problematic (and had been causing problems) as a sprout... them becoming a mentor only made it worse, because now they have kick powers. So it's the person, not the crown.
Also on 'most' mentors being 'rude', generally that means either most helpful mentors you met just have their crowns off in the overworld (a common practice because of the 'mentor bad' meme), or you are being rude to them first or otherwise took what they thought was neutral as being negative. Usually it's more complex than that though.
Other people have already addressed the points regarding 'watch videos!' &c (see: Eien713's response), so go reaed their replies on that matter. A quick TL;DR is that if you're doing hard content (extreme/savage) you should have read guides not just on the fight but on your job and role as well. For casual content (dungeons) it's fine.
There's only so much mentors can teach, and it's beyond us to push you from 'doesn't know anything about their job' to 'decent and workable' in one lockout... and honestly, very few people have the patience for that. The few people that do are a treasure.