I actually think this is exactly the point of the scene. Warner clearly thinks of Marilyn as gorgeous but dumb and Jackie as smart but less sexy. But we’re in this movie from Elle’s point of view. And Elle doesn’t understand Warner’s implication here because, to her, both these women are successful and beautiful and amazing. The only difference is one is blonde and the other is a brunette.
This scene isn’t trying to make Elle look stupid, because the point of the movie is that she’s not. She’s just incredibly, unabashedly feminine. And Elle’s way of thinking here is right. The main difference between Marilyn and Jackie, that Warner would know of and therefore could be alluding to, that is also something Marilyn and Elle have in common with each other, is their hair colour.