First Lady Michelle Obama may be America’s brightest star—outshining leaders from Washington to Hollywood. In a favorability poll conducted by Gallup in July, she topped the list, ranking higher than Sarah Palin, Hillary Clinton and Al Gore. Last month, CNN placed her approval rating at 65%—20 points above her husband’s—and Forbes named her the most powerful woman in the world. With every TV appearance or press shot, she moves markets. Research by New York University business professor David Yermack, published in this month’s Harvard Business Review, reveals that Obama’s style choices have contributed $2.7 billion to the retail sector. Companies...