BDD (Body Dysmorphic Disorder) is a mental illness that causes one to see either entirely imagined uglyness in his form, or exagerate real flaws, to the point where it becomes difficult for him to function socially.
A few notes in Kafka's diary seem to point to the direction that he might have had some variant of this illness. Also in at least one of his letters to Felice he claims that he is ugly, but that this might be just imaginary, and that she might even think he is beautiful.
BDD could explain some of his tragic life, since it is associated with depression and even suicidal ideation.