I suppose though, looking at these comments, you're classing "stupid" as someone who can't formulate an opinion on anything, which I guess would be pretty dull and unattractive.
http://notalwaysright.com/category/cant-be-that-stupid: a refresher course on the flavors of stupidity.
For me, though, mass ignorance of basic common knowledge (stuff that is part of the typical education curriculum) and the inability to learn is another turn-off. Synonyms: thick as a brick, dimwit, halfwit, nitwit, numbskull, airhead, nothing-between-the-ears, bimbo, blockhead, simpleton...
EDIT: *sigh* blah blah, intelligence level has nothing to do with your worth as a person. I know. But we are talking about relationships, and I would prefer to be able to discuss my thoughts with someone who would be able to understand them and engage in meaningful dialog.