Aesthetics speak. Craigslist says "cheap". MySpace says "My parents wouldn't like this place and I can decorate it how I want". I don't know what plentyoffish says.
Sure. But the word you're looking for is functional, not ugly. The ugliness is just a side effect.
Craigslist was criticized for being too plain from the moment it was built. But it's buried a lot of better-Photoshopped competitors.
Dove is using this idea in their "Campaign for Natural Beauty" (http://www.dove.us/#/cfrb/) . The models aren't ugly, but they also aren't the usual perfect-looking models.
When I was in grade school, I remember there were anti-smoking and tobacco posters on the wall of severely aged, sick looking people along with some text about how "chewing cost me my mandible". Those were examples of actual human ugliness, and they worked.
So that's all to say, ugly could be part of your message. Which isn't really a novel idea.
What's more interesting, in my opinion, is when something seems bad from a usability standpoint and still succeeds. plentyoffish.com fits under this category.