The phrase "is down mouth" means to have a gloomy, depressed or negative expression on one's face. Antonyms for this phrase would be words or phrases that depict a bright, happy or positive expression. These can include lively, cheerful, optimistic, buoyant, and upbeat. Other antonyms can include radiant, glowing, shining, sparkling, and lively. These antonyms can be used to describe a person who is full of energy, life, and happiness, and is portrayed on their face with a big smile, glowing complexion, and a lively demeanor. Such happy expressions can potentially spread positive vibes to others, making them feel better too.