Smarten up is a colloquial expression used to describe someone improving their appearance or behavior. Some common antonyms for smarten up include slovenly, unkempt, disheveled, and dowdy. These words convey a sense of untidiness or carelessness, and imply that someone has not put effort into improving themselves. Another set of antonyms for smarten up could include devalue, diminish, or worsen, all of which imply that someone is not improving themselves or their situation. In short, there are many words that can be used as antonyms for smarten up, all of which convey a sense of neglect or lack of improvement.