I never had a single experience of that in 130 hours of online in Tri, best online community ever for me.
Veteran hunters were happy to help newbies on early quests as they were there to farm materials, and then I did the same later on. As long as you check out exactly what each group are after you'll be OK, expecting three veterans to sign up to continuallt do all of your low level quests for you while you just turn up to carve the beast at the end is a bit poor, especially if what they want to hunt is something you aren't allowed to join because of a low HR. The best thing to do if you're struggling is to join hunters who are just slightly above you, that way you all still get a challenging hunt and you aren't stopping them taking a full-strength team on a high-level hunt.
If it's veteran hunters farming bosses you can only just access, you aren't going to be much help and you aren't going to learn much as your equipment means that you are a liability, a one-hit kill when you only have three lives between four of you and they might be waiting for a more powerful player. The best thing to do is to start your own room, listing what you want to hunt, and get going. People of a similar level (or experienced hunters happy to help out) will join you soon enough and you can all learn together.
There's a fair bit of etiquette in Monster Hunter- take turns in posting quests, leave a room if the other three are farming a quest over and over (one of them is after a very rare drop). If you want help with something you are stuck on, start your own room so you can say so when others arrive. If it's quiet, join a room and sign up to help the others first, then post your request. Don't just arrive and immediately post your request, as the others are probably mid-cycle in helping each other- another hunter may have been patiently helping others for an hour, waiting for help with their own task. Having a new arrival run in paste up a request isn't going to get the other three to join, as they already know whose turn it is, and it isn't the new guy.
It only takes one decent hunt to get a crowd that will farm with you all evening, it's just about putting others first for a change, and not expecting a room to immediately change what it's been doing because you need help. We once had a new guy beg for help with the last boss of Tri online, Aleotron. We went in, and this guy stayed in camp for the whole battle. Another hunter had server issues and disconnected. That left two of us, half-strength, to do a long, tough battle for a player we had never even seen before. It took 35 minutes, we almost failed, and he stayed in camp the whole time. When he finally ran out to carve, each of us kept interupting him as we felt he didn't deserve the items as he had made no effort to learn the battle. He was so pissed off, but to be honest so were we with him. We kicked him immediately after too, but only for taking the piss, not for needing help.
Considering the amount of racist, bigoted nonsense that other communities can spawn, calling monster hunter toxic because high-level hunters don't want a quarter of their strength to be a liability when farming tough beasts seems a bit inaccurate, especially for a game where you can play for 45 mins and end up with nothing if a player dies three times.