Perhaps because Ducky comes off as being rather cold and stuck up. Missy, on the other hand, may have been conceited as all get out, yet she was always very outgoing, and theatrical, and she really did care about others, and she was extremely generous.
As for Ducky being a beauty-no. Bach, I'm with you that the book was pretty much a white-wash. Missy was the beauty in the family, not Ducky.