Yep, making her Japanese would've been even better. But knowing Hollywood, they wouldn't have cast a Japanese actress for the role. Maybe Rinko Kikuchi, but that's about it. Remember when the main female cast of Memoirs of a Geisha were Chinese actresses? As long as it sells well, they don't care about casting what we would believe to be the right choice. Scarlett Johansson is a well-known actress in the west (mostly due to her looks, which aren't THAT great imo) and of course with a famous actress in the leading role, they're practically assuming that it'll be a hit anyways and also aim for those not familiar with the franchise.