"Mules in horse harness" can be used to describe a person who looks pretty, modern, and even elegant, but is actually selfish, snobbish, or lacking in elegance of the mind.

Said in Gone with the wind by Mammy to Scarlett O'Hara, after the latter had done many things that could not be accepted by the society, including marrying Rhett Butler, a rich outcast of the society.