<< was she simply to tired (or distracted by anti-Mexican bigotry) to take care of daily grooming tasks like brushing and combing? >>
Middle class women's hair in the 1970's was often teased into shaped curls and waves. It was kept in place by day with hairspray, and with special non-stick tape crisscrossed across it for bed. If you had money you went to a beauty parlor weekly to keep the style in shape. If you were poorer, you sometimes wore wigs during the day (As synthetic hair holds a styling longer than human hair.)
I'm assuming that since the mom is a poor emigree with upwardly mobile aspirations, she wears wigs so she can have popular hairstyles with minimum expensive upkeep.