There are 2 different types of beauty.
IF one is talking about skin, hair, teeth, eyes, etc. then that is the skin-deep beauty, yes. IF this is the topic of a book, then the person on the cover should properly represent these desirable attributes.
IF the book is about INNER beauty, then there is no need to have ANY person's picture on the front of the book. Just a picture of nature, an abstract drawing, or even a solid color with text only.
It is okay if people are not beautiful on the outside - but those people should not enter the "Mrs. America (outer beauty) Contest"!