The point of the 'ugly daughters' is to point out that women's value is not skin deep.
As another person mentioned below the two step sisters have different qualities. You might have noticed how Ashley is clumsy, and she just can't do anything properly to save her life, but she is a kind and, yes, beautiful girl. However, the two sister have so many qualities, they are very good at housework, one is intelligent, the other is artistic, one is perceptive, the other intuitive, they can acquire men's attention with their actions rather than their appearance.
And honestly, in real life, how many times have you met people who were not considered 'beautiful/handsome' either by society's or your own personal standards, yet you came to love them for their charisma, intelligence and/or originality? In my case, it happened a lot.