I have not read the book but I can answer this question. I had a friend from Korea who lived with me for several years. She had a son. One day she explained to me that in her country when a mother has her first son the mother is no longer referred to by her name. Instead she is called and known by her son's identity. My fiend Kim's son was named Li Kin and so she was referred to by her friends as Li Kin's mother.
My sister who is half Japanese also had often heard her mother referred to as Kim's mother even though her name was Yukarie. Kim was her mother's first born son.
Calling a mother by her son's first name is a sign of respect and tradition in many countries.
She was always called Nwoye's mother because Nwoye is her first-born and in most societies the mother is always called after her first-born son.
hi, i'm accually studying this novel and i think i have an answer for you. there is no explicit quote or sth that can testify my answer but anyway ill try. so as u may know, women in "Things Fall Apart" are not really respected; being a woman is being weak, feminism is always related to sth negative. women are almost never called by their names accually, but by the status they occupy in the society so a woman is more or less respected when whe give birth (her husband may sacrifice a goat for her if she gets 3 boys succesively). so to answer your question, Nwoye's mother is not called by her name, because for the society it's more imoprtant being a mother than a wife or simply a woman.