The term "did one's thing" refers to performing an action or task successfully. However, one can also fail to do their thing, which can be described by antonyms like underperforming, faltering, or floundering. Another antonym for the phrase is "did nothing," indicating a lack of action or accomplishment. On the other hand, an overachiever or a perfectionist can be described as overdoing their thing. In summary, possible antonyms for "doing one's thing" include underperforming, doing nothing, faltering, floundering, and overdoing it. The choice of antonym depends on the context and the degree of success or failure in completing a task.