Geek girls stick together, and look for themselves in their entertainment. They will pick up on the brainy computer whiz kid actually being a girl for a change. They will cheer when a woman becomes Chief of Engineering. If you know what's good for you, you will cheer, too. So you finally get to meet your geek girl in person.
Most geek girls have low-maintenance appearances.
More importantly, however, is the comfort issue. Jeans are the staple element of the geek girl wardrobe. Glasses, usually strong, are also a familiar touch. They don't dress for success. They dress for comfort, for long hours in the lab, for convenience.