Even if you do have half your face burned off a la Two-Face in the Batman series, being visibly smart and funny will boost your apparent prettiness by quite a lot.

I find that most people have some things attractive about them. If they are interesting and kindly disposed toward me, it is not hard to focus on the attractive features, and blur out the less attractive features. It works very much like the affective death spiral, but with no real negative consequences.

Once you find enough things attractive about someone, you enter the spiral, and you begin to notice the very attractive square line of Harvey's non-burned jaw, and just don't even notice the scary skeletor burn face anymore, or you might even find little parts of it that start to look interesting to you.

Well, this all assumes a counter-fictional Harvey that doesn't go fully dark-side, or recovers at some point to something like his former moral and mental self.