What metaphor is used to describe Mae Tuck's appearance?

Mae Tuck is compared to a potato.

A metaphor is “a figure of speech in which a word or phrase is applied to a person, idea, or object to which it is not literally applicable” (Guide to Literary Terms).  In this case, when we are first introduced to Mae Tuck, a metaphor is used to describe her.

Mae is compared to a potato.  The figure of speech is a metaphor because the author does not say that Mae is like a potato, she says that she is a potato.

Mae sat there frowning, a great potato of a woman with a round, sensible face and calm brown eyes. "It's no use having that dream," she said. "Nothing's going to change." (Ch. 2) 

Of course, the author does not mean that Mae is literally a potato.  She just means that she shares the characteristics of a potato.  Both the potato and Mae are round and brown.  This figure of speech helps us picture Mae Tuck.  It may not seem very nice to describe someone like a potato, but the author does not do so maliciously. 

Mae Tuck might feel a little old and wary, because she has been immortal for over eighty years.  Mae does not age.  She does not even need to look at herself in the mirror, because her reflection never changes. 

Mae Tuck didn't need a mirror, though she had one propped up on the washstand. She knew very well what she would see in it; her reflection had long since ceased to interest her. For Mae Tuck, and her husband, and Miles and Jesse, too, had all looked exactly the same for eighty-seven years. (Ch. 2) 

Mae is excited because she gets to go back to Treegap. She has not been there in ten years.  She and her husband have two sons, Miles and Jesse, and they rarely get to see them.  The boys became immortal when they were still young, and they like to travel.  Jesse was seventeen and Miles was twenty-two.  They will always be those ages.