A group of fed-up restaurant workers yesterday took aim at Mayor Bloomberg for touting that he was endorsed by their boss — whom they’re suing for allegedly cheating them out of wages and tips. Lawyer Justin M. Swartz, who represents the Mama Mexico workers, blasted Bloomberg’s praise of owner Juan Rojas Campos on his Web site. “Mr. Rojas Campos’ ‘American dream’ story, as Mayor Bloomberg calls it, isn’t very American at all,” Swartz said.