I’d say that if faction mentality was ever a thing it was in the old days of MMORPGs, if ever. I never really bought into it though, I remember hearing people QQ about the other faction all the time in the old days.
Everyone has a particular fantasy that tickles their fancy, in Vanilla that may have been more pronounced due to shamans v paladins, good v evil, orc v human, etc, but neither side is more inherent to being bigger crybabies or losers or whatever. Whenever I played Alliance it was the same thing you saw on horde. I haven’t played Alliance seriously since MoP though.