The phrase "cutting mustard" means to perform exceptionally well or to meet a high standard. Antonyms for this phrase would include phrases such as "falling short," "missing the mark," or "underachieving." These antonyms indicate that an individual or group is not meeting expectations or is not performing at the level that is required or desired. Other antonyms might include phrases such as "failing to impress," "not up to par," or "disappointing." These antonyms are often used to describe situations in which individuals or groups are struggling to meet expectations or perform at the desired level.