It should be obvious from the above that I don't share Mencken's unflattering view of the masses. I think, if anything, his "plain folks" are
[WS:] You need to put him in a historical context - his wrath was directed mainly at the anti-immigrant mobs of Baltimore (if I remember correctly, his father was a victim of these mobs), "ku-kluxers" (as he called them), lynching mobs, the religious mobs (Scopes trial) as well as at politicians and other "mountebanks" who thrived on mob appeals. Like Mark twain, he was more against petit bourgeoisie than against "ordinary people."
As to the notion of the "masses" - it is one of those propaganda concepts created by populist demagogues that is high on emotional appeal, but has virtually no empirical meaning and it obscures more than it can possibly explain. Moreover, it is a very derogatory and arrogant notion that lumps people into an artificial category, regardless of how these people actually think and identify themselves.
Wojtek