In her 2016 book, Thought in Action: Expertise and the Conscious Mind, Barbara Gail Montero, a professor of philosophy at the City University of New York Graduate Center, observed that, “Although we praise effort, we prize effortlessness.” A former professional ballet dancer, Montero was referring to the feats of athletes and dancers, as well as musicians and other artists. But the sentiment applies neatly to the way we evaluate beauty and style: In magazine articles, on social media and in countless SEO-friendly blog posts, insouciance is always in fashion.

“Effortless style” is Jane Birkin in jeans and a men’s shirt, with a straw pannier on her arm; Carolyn Bessette-Kennedy’s ’90s minimalism; and virtually every photo of Zoë Kravitz. It’s Glossier’s paradoxical no-m...