Gunna add my .02, and some recent examples as well.
First and foremost, I strongly feel that preparation for higher end content is an absolute must. Being generous, lets qualify high end content as Susano, Lakshmi, Shinryu, Byakko, Deltascape Savage, Sigmascape Savage, and Ultimate Coil.
I cannot tell you how many farm parties I have joined in the last 2 weeks alone (for everything above except Sigma/O4s/Ultimate) where the Party Finder is called a farm party, and we struggle to get one clear. Yes, even on Lakshmi. Yes, even on Byakko. I find that sometimes, the excuses people come up with are ridiculous. Ninja died repeatedly on one Byakko run last night "because he was tired"... why not just go to bed then? If you are going to play so horribly in your present state... dont hold the rest of us back. Before you queue up for/join a party for certain content, evaluate your present mental and physical state. Are you focused? Are you watching netflix because you were just crafting? Have you been in PotD for the last 9 hrs? Overall, I find that a lot of the playerbase "think" that they can get something done but arent being honest with themselves over their mental preparedness at a given moment to actually participate in a meaningful way.
2nd, and arguably most importantly, you have to be honest with people about your experience level with a given encounter. My static (recently formed, and still on O5s) was supposed to raid last night but we had multiple people unable to join. So the remainder of us decided to do Byakko farming. 3 of us (static members) have cleared the fight together about 40 times and probably even more without each other. The other members of the team stated they had cleared it. Then, the PF fills. We go in, and its a complete shit show. Come to find out one of my static members "got carried" through it, and didnt want to not be able to join us. No one was mad, and no one harassed this player, but I think it is only fair that you are completely 100% up front about your experience in an encounter before you waste other peoples time. I think the reason people dont do that is because they are afraid they wont be able to be in a party. I have news for you... I would much rather have you say "ive cleared it twice, but Im still confused about X mechanic" so I can give you my discord info and call out mechaincs/tell you about them. I think most rational people will be willing to help, if it means a smoother overall experience.
As to what can be done about it, sadly... I feel it is completely on the individual to acknowledge their shortcomings but it is also on other members to NOT BE COMPLETE CLOWNS and actually help people out.