Elon Musk mocked it as the “Shortseller Enrichment Commission.” Billionaire Mark Cuban said it’s “useless.” Hedge fund legend Leon Cooperman called it “abusive.” For the U.S. Securities and Exchange Commission, such attacks come with the territory. But brushing them off is getting harder in the age of social media. One online foe has so troubled the agency that it’s made the remarkable move of seeking to hire a reputation-management expert to burnish its image.