I think "Not another teen movie" addressed this (as well as a bunch of other cookie cutter type plot points) extremely well. When they talk about who they are picking for the bet they have comments like, but she has paint on her jeans and glasses, while refereing to the conjoined (at the head) twins as being too easy to transform.
I think they did a better job making Hatheway look unpretty than they did with Bullock. But either way, these movies always use someone that is beautiful to begin with, so they have to 'uglify' them by putting their hair in an unflattering style, and using makeup to do the opposite of what the makeup usually is used for.
Besides, these movies are more about (or are supposed to be, as this is the message they try to get across) the inner beauty of the person. At least thats how I always figured it, i mean it is a 'Cinderella' story, and is that not what Cinderella was all about?
-Darth Simon