Deformity, that is, Richard's deformed physical body, is a symbol of corruption: orruption of the mind and soul of the individual, but also corruption of the kingdom Richard inherits.
Richard thinks his ugliness is repulsive, but despite his appearance, he uses his charisma and powers of persuasion to harm others. He seduces Lady Anne, even after he has admitted that he killed her husband. She tells Richard he is a "lump of foul deformity" but still agrees to be his lover. Even as Richard's deformed physical self is what he rails against, what he says holds him back from his ambitions, he obviously is able to win a beautiful woman, proving that it is not his looks that make him "ugly," but his thoughts and deeds.