For sure, this is true.
OP, I think that the reason you find the nicest people in that broad middle range has several contributing factors, but keep in mind that there are jerks (and very nice people) at every stage/type/difficulty in WoW.
1.) The Disaffected Solo Player
(Not to be confused with “all solo players”)
People who spend a lot of time in M0’s, LFR, and queued world content are far more likely to be solo players. Most of WoW’s solo players are wonderful people who play alone because they don’t have time to commit to a guild or raid schedule, because the people they were guilded with in previous expansions no longer play, because they prefer solo grinding old achievements and this has always been their endgame, etc…BUT: The players who can’t get along with other people well enough to be in a guild/grouped content on a regular basis are also solo players.
These are the people who can’t get others to put up with them for very long, and these are the players who can give you a long list of sob stories about how awful everyone else is, how skilled they are, how “nice” they are to everyone, and it’s always everybody else’s fault. They will blame people, rant, toss insults, and leave groups only to harass and harangue whoever made them rage quit with abusive tells until blocked.
They have never been wrong. No one has ever suffered as they suffer. No one has ever recognized their genius, and they could outplay any Arena or MDI all-star if someone would just give them a chance and they didn’t have to put up with all of us noobs.
The reality is that this player simply cannot consider the feelings and needs of others, and he/she is totally blind to his/her own faults, gameplay mistakes, and attitude problems. For all of these reasons, such players tend to stay down in the low-end content and have difficulty accessing groups and guilds that would include them in anything else.
2.) The Retired Elite Raider
(not to be confused with “all mythic raiders”)
These are the people who used to play bleeding edge content, but have gotten older, moved to a new stage of life, or dipped because they no longer wanted the stress and commitment of high-end PvP or raiding…but they still operate within their new guilds as though they were playing at that level.
The truth is that hard modes and top content has gotten increasingly difficult as the expansions pass, and what constituted the cutting edge 10 years ago has been surpassed, but these are the players who either don’t understand they’re not as good as they once were (reflexes dull and lack of use will rust the gears) or they don’t accept that today’s endgame is not like yesterday’s endgame.
This is the guy who will excoriate people for small mistakes, pontificate ad nauseum about the sovereign importance of min/maxing in classic or heroic level retail raids, and throw around insults at anyone who enjoys mid-level difficulty and has no aspiration to cutting edge achieves or going beyond KSM.
3.) The Contented Middle
(not to be confused with “everyone who doesn’t mythic raid or LFR”)
The people who operate in the middle have no pressure on them to perform at peak, but they also get to experience all the content they aspire to participate in, and they typically run with a group of friends or an established guild. These is the population most likely to be chill, empathetic, and forgiving when mistakes or even disasters happen.
Again, there are jerks and absolute gems in EVERY level of play in WoW. I have met CE raiders who are willing to teach, have an excellent sense of humor, and demonstrate loads of patience with “bad” players. I have met solo players who are an absolute delight in group content and on comms but have their own reasons for not wanting to guild. I am guilded with a lot of people who used to play top end content but no longer do and still manage to treat everyone they encounter in the game with kindness and patience.
But in answer to your question about why people in the lowest PvE content tend to be more overtly toxic? I think my observation is at least partially true part of the time. There are lonely and discontented folks who don’t know how to get along with other people and cannot be bothered to try, and most of those types get relegated to the lowest difficulty content.