“The perfect mother” is a cudgel to cut down the flesh-and-blood variety — By Miya Tokumitsu I n 2014, the advertising firm MullenLowe launched a campaign, “World’s Toughest Job.” The ad firm listed a fake job, “Director of Operations,” in newspapers and online, and held interviews with a variety of hopefuls. The inter-viewer then went over an extensive roster of requirements: working on…