Remember that one time when Topanga took out a pair of scissors and started doing quite the hack job on her gorgeous hair in the hallway at school? That is by far one of my favorite episodes, and it's always stuck with me.
Topanga was trying to teach Cory a lesson about having a healthy self image, and that what's inside is much more important than what's on the exterior. Of course, as soon as she chopped her hair into a blocky mess, she found out the hard way that she valued appearance much more than she ever thought. Like Cory said, it was easy for her to say beauty is on the inside, because she never had to worry about not being beautiful on the outside.
By the end of the episode, everyone learned that it's a good idea to not get too caught up in what you look like, and that what's really the most important is the person you are on the inside. Who you are is what matters; not the color or cut of your hair - or whether or not you look like Mr. Potato Head.