The guy is giving out strange compliments to girls (in an effort to be funny).
For the first girl, when he says, "You look like you wore your retainer," he's saying that she has nice looking teeth, or maybe that her smile looks nice because her teeth are straight. The retainer is a device used to help keep teeth straight.
For the second girl, when he says, "You look like you'd never hit a curb," which I think means he's complimenting her on her driving. The curb is the raised edge of the road and when you park your car, you don't want to hit it too hard or you can screw up the alignment of your wheels. Of course, how would he know if she was a good driver just by looking at her.
Yes, it doesn't make much sense, but like I said, he was trying to be funny by saying some ridiculous things that you normally wouldn't to compliment a girl.