Pretty up refers to making something or someone more visually appealing. A few antonyms for the word pretty up include uglify, mess up, and disfigure. Uglify means to make something look unattractive or ugly, while mess up means to make something untidy or disorderly. Disfigure refers to making someone or something lose its original shape or appearance. Other antonyms for pretty up include dull down, tone down, and plainness, which refers to the state of being unattractive. While pretty up may be a positive action, its antonyms usually result in negative consequences and should be used with caution.