Does the American electorate have a distaste for plainspokenness?

[Simfan34]

President Adlai Stevenson agrees- Americans love their eggheads.

No. Americans have a preference for "straight talk", "no-nonsense", "common-sense" politicians. People generally like people who say what is on their mind and simple things on their mind. Your examples didn't lose because "they were in the mainstream". They lost because they weren't in the mainstream, or in Dean's case, because of a scream.

What you call "plainspokenness" isn't really such. It's "not insulting those whose votes you need to win". Boris can do it because he has no intention or expectation of having those he insults vote for him, not at least the Labour councillors who he called "great supine protoplasmic invertebrate jellies" (I assume that's what you're referring to). Nor do Obama, Clinton, or Walker have policies that "offend large sections of the populace", at least not ones they've bothered to conceal. Everyone knows Walker is not a fan of labor or that Obama likes his health care reform.