I think I'm finding that certain sims have children that look better with some good looking sims than with other good looking sims, for example, I've found Steel-Jolina children to be fairly average, but for thornton Wolff - Jolina children to be good looking. Also, I've (personally found) that Steel-Morgana Wolff children look very good and better than full Wolff children, but this could all just be down to chance in my games. Also, obviously, Mortimer and Bella's genes mix well.