Intelligence is sexy. Lack of a personality is unsexy. Lack of a personality reduces the sexiness of intelligence by way more than the amount provided.
So:
Sexy:
Confidence
Intelligence
Humor
Physical appearance
Unsexy:
Lack of confidence.
Ignorance and stupidity
Bad sense of humor.
Ugliness.
Unsexiness in any of those categories has higher weight than sexiness. Take that as you will.