Most Helpful Girl
Well I'm a girl and I can answer that because I've dated girls before. There was maybe 2 pretty girls I knew who had no personality. They were like sponges and only mimicked what they saw. Didn't have anything to offer and had low intelligence but was pretty devious and manipulative. I've met some pretty people with great personalities as well. You can't match a face to a personality. It's just one if those things you have to get to know.3
Most Helpful Guy
There is some truth to it, otherwise the stereotype wouldn't exist. However, I wouldn't call it a "rule".
Have I met attractive girls that had the personalit of a brick wall? Yes. There was a fair amount of those girls at my school. Granted, I went to a school full of affluent daddy's girls, so I think my case is unique. There were some girls there that were really hot until they opened their mouths, and there are some that have no life outside of modeling or working at Starbucks. It's unfortunate but it happens.
However, I have met many attractive girls that break that idea. Just because a girl is attractive doesn't automatically mean she lacks in personality or smarts, some girls really have it all. There are a lot of pretty girls out there who do not know they're pretty. At worst I'd say I met more pretty girls with bitchy personalities than "dull" ones, but I've also met a lot of unattractive women who had bitchy personalities, go figure.
So I say don't just assume a girl is dull just because she's attractive. Wait until she starts talking about something and then you'll find out.1