Do you mean significantly less attractive or not though? I mean, I know there are some people who are absolutely stunning, but I know as a girl I'd be scared to even go and talk to a guy who was, so guys might feel the same about girls. But if you're just talking about two girls with above average looks, one of whom you feel is slightly more attractive than the other, the guy might think differently.
Also, yeah, what people said about confidence- always seems to attract people, no matter what the girl looks like, just because she's happy and comfortable with herself, and that makes their personality more attractive I guess...