At my school, we've all kind of accepted that we're really big dorks, so there's isn't a single sorority that is *gorgeous.* Everyone has their really pretty members, their average members, etc. I agree with what everyone else has said so far - average looking shouldn't keep a girl from being considered for a bid.
I understand the importance of keeping up an image, but at the same time, peoples' looks can change so much. Speaking from personal experience, I know pictures of me from freshman year are pretty scary.
Some people eventually stop taking care of themselves, and their looks may go down, while many others picked up pointers from their sisters along the way and looked even better than before!
One of the first things we did as a pledge class was have a makeover session (one of our sisters was a Mary Kay rep and was looking for practice, so we volunteered as a NM activity). Not only was it a lot of fun, but with a little makeup applied the right way, some of the average girls became gorgeous! Plus people like me who can't live without makeup got new tips and got to try new colors had fun too.
I'm not saying you should invite her into your chapter and try to change her right away, but after spending time with a large number of girls, stuff like that tends to rub off and suddenly everyone starts improving. My one sister turned me on to Ann Taylor, and I haven't looked back since. I taught another how to straighten her hair. We all pushed each other to work out (not in the hazing sort of way, but because we all wanted to - and everyone has those, "I don't want to go to the gym" days). Self-improvement is one of the biggest benefits of being in a sorority, in my opinion, and also the most fun!