They must have money to waste. She's gorgeous on screen but not very imposing in print, and for someone who seems laid-back in her style to suddenly feign interest in ten-step skincare regimes and expensive lip balms... it'll be one the biggest acting roles of her life.
For someone to be a successful spokesmodel for a beauty brand, there first has to be a compelling reason why the public would look at the person's appearance for inspiration. One decent cover out of every ten she does (as herself) isn't it.