Other forms: disfigured; disfiguring; disfigures
To disfigure something is to ruin its appearance, which is what would happen if you drew a big bushy mustache and a pair of antennae on the Mona Lisa.
Unfortunately, disfigured people are almost always portrayed in the media as villains, or victims. The word does suggest a negative view of someone (after all, it doesn't just mean changing way something looks but specifically spoiling it). Organizations for disfigured people, however, like Changing Faces in England, have claimed the word as their own and use it proudly.