So... let me break this down into what seems to be several complains, because all of them warrant a different response.

1) "Its my first time in this dungeon"
Dungeon in this context might be replaced by anything that isnt a Savage or Extreme Primal duty or in other words "casual content".
Those people should indeed not be belittled or mocked or in any other way robbed off a pleasent gaming experience provided they are doing two things: Playing their job appropratly for the given level of the duty in question (that means its okay for a tank to lose aggro in Sastasha during their very first run, but NOT okay for them to lose aggro in The Burn at level 70 - similar things apply to healers and dps, obviously) and that they either pick up mechanics fast enough by themself or listen to advice thats being presented to them in a polite manner.
Being new absolutly shouldnt mean getting harrassed, but it also includes the new person showing respect for the time of the other 3 people in the duty by trying their best instead of hiding behind "I'm new!" - this goes especially for people not being able to perform the basics of their jobs in endgame-content and then excusing that with being new to the dungeon.

2) Mean Mentors
This very much depends on the duty you get for your mentor roulette - as we all know it has a bunch of extremly complicated Ex-primal fights in there that are pretty much impossible to clear unless the majoruty knows the mechanics. And a fight like Thordan EX can hardly be explained during or before the run. Again, not a cause for harrassment but watching a video and informing yourself about those fights before queuing up is pretty much required. Sadly, the game does a very, very poor job of communicating to new people that EX-primals arent something they gonna clear using the DF. Not even with mentors. While I personally try my best to help them clear those fights, I honestly feel the two best advices you can give a new person in a duty like that is "Do this unsyched by using the PF" or "If you dont wanna do that, watch a video before you try this again".
Obviously this doesnt apply for normal duties, because they're so simple you dont need to watch videos in the first place.

3) Healer not dpsing on the first day of 24-men
Being anxious to a certain degree and more careful is understandable, but every healer has enough tools that allow them to quickly heal damage that occurs. Not knowing every mechanic thats gonna happen is not an excuse to stand around during those fights doing nothing most of the time. Being more careful - yes. Outright refusing to do something but stand around for a minute, waiting for the next thing to happen? Less acceptable, even on day one.
Like I mentioned earlier: This person is new to the content, not their job. I get wanting to play it safe, but thats no reason to basically not play at all.

4) Being "funny"
Humor is subjective and might not translate to well in a chat. Quite frankly, I find comments like the one you described making nothing but incredibly annoying and not the slightest bit funny. Call me humorless, but personally I want nice, clean farms, not some clown trying to "break the ice" with a comment thats doing nothing but raising red flags for people who have been burnt in the past by having people sneak into their farm-parties without knowing the fight.
People might not be paying attention to your weapons or to messages like that on the screen and they shouldnt have to just to notice something you consider a "joke". Joke around with people you know, dont try being funny with people that you dont and therefor have no idea how they're gonna react.
People in farm-parties might have been doing this content all day, are exhausted by now, just want their clean runs and are therefor a bit more sensitive, when it comes to "jokes" like that - and would rather run this content with someone who takes it more serious. Yes, its a game, yes, its supposed to be fun - but you need to accept that fun doesnt has the same meaning for everyone. Silly jokes might not be fun to the farm-parties you join.

5) Actually entering parties without the knowledge they're asking for
Obviously a simple No-no. Dont do that. Ever.
Dont enter a farm party if you dont have a few clears under your belt.
Dont enter a clear/enrage party if you havent seen the fight all the way through.
Dont enter a "practise phase X" party if you cant reach that phase comfortably.
If you're completly new to the fight, join a practise party from the start.
The "law" for this is the requirement that the party leader set for their PF.



Its not a crime to be new as long as you're trying your best and are being honest about it and respectful towards your fellow party-members.
Being new obviously doesnt mean that harrassment is okay - of course not!
But it also doesnt strip that person of any and every responsibility. Certain content does require a certain preperation and certain knowledge and skill has to be reasonably expected at a certain point.

We all need to treat each other with respect - the vets the new people by helping them the best they can; the new people by trying their best in return... and everyone by not making stupid comments that might be received the wrong way and cause unnecessary drama.