I'm not sure you're right about Mencken. I have the impression that he was more of a cynic than an elitist, not a groveler before wealth and power. There's a series of interviews with him on YouTube, done by a protégé of his as a kind of oral history. I dabbled with listening to them a few months ago and remember one anecdote. Mencken described how he enjoyed walking through neighborhoods of Baltimore at night, looking into well-lit living rooms to learn what he could about how people lived. For the most part he thought people's taste was appalling, but he mentioned that one poor black woman's house had struck him as being arranged with some of the best taste he had ever seen. I don't know that much about Mencken but my guess is that "good taste" was high praise.
About Buckley, on the other hand, I agree with you. I've watched some of the interviews he did in his prime and seen nothing but an oily, pretentious jerk, sure that his establishment platitudes are convincing arguments. Those pieces really have not aged well. I'm thinking of the ones where he "debates" Chomsky ("I'd smash you in the goddamn face") and, infamously, Gore Vidal ("Now listen you queer, stop calling me a crypto Nazi or I’ll sock you in the goddamn face and you’ll stay plastered.") Some intellectual hero.