[My Dreams killed me]

My thoughts are that often nice people often also come with the personallity quirke of either being shy, or having certain issues with confidence or there ability to speak gracefully. Sometimes people can take this third situation as stupidity, when it really isn't the case.

On the other half of the spectrum. People that can be seen as blunt often also contain the quality traits such as charisma, humor, and easygoingness. Sometimes people are attracted to bluntness, seeing it relates to honesty. Nice people have a tedency of being over-nice, or trying not to offend someone and may often lie to make the person feel better, which may lead the to the impression that they are "sucking up" or just plain "not being truthful" Mean people who arn't assholes (people that are mean just to be mean) can often also be seen as very truthful. People that arn't offended easy can get along faster with mean people than "suck ups".
Of course that's just an observation, I don't really have evidence to back any of this up.