Can we not form a causal link between skill level and toxicity? That's as bad as saying maturity level scales with physical age. And as anyone who has watched a Friday Night Magic tournament will tell you, 40-year old children do exist.
There are toxic good players just as there are toxic bad players, and there are polite good players just as there are polite bad players.
Generalizing all bad players as being toxic only serves to generate a stigma that every player who doesn't perform up to snuff is a toxic player, which in turn will only serve to scare away the nice players who are more than willing to accept constructive criticisms.
Note that I say CONSTRUCTIVE criticisms. Just telling someone "You suck, git gud" isn't an eye-opening revelation and it certainly doesn't give a bad player any advice they can put into application.
Like... I was in an Aurum Vale run for my relic a while back. And in that run, there was a tank who was, like, a Level 47 DRK who admitted to being new to the job and had Level 40-45 gear on his person. The healer and the other DPS were shitting on him so hard for constantly losing hate despite the tank doing everything in his power to hold it, the DPS never using Quelling Strikes, and the healer never using Shroud of Saints to reduce their own aggro. And not once did they give him any advice, nevermind any advice that he could actually apply as a tank. It was all "OMG, you suck. You should uninstall the game. What a horrible tank."
tl;dr: There are toxic people everywhere, and it has absolutely nothing to do with skill level.