The omitted Tyrell woman who is Mace Tyrell’s wife and Margaery’s mother in the novels is Alerie Tyrell.
Alerie was born Alerie Hightower, making her a member of the powerful Hightower family of Oldtown. She marries Mace Tyrell and becomes Lady of Highgarden. Their children are Willas, Garlan, Loras and Margaery, placing Alerie at the center of the Tyrell family’s dynastic influence.
In the books, Alerie joins Margaery’s entourage when Margaery travels to King’s Landing to marry Joffrey Baratheon. She is described as tall, with long silver hair, and leaves the capital with Garlan after Tywin Lannister’s funeral. Unlike Olenna, she is not a major viewpoint or political player.
The television series removes Alerie entirely and gives Olenna a much more prominent role as the family’s sharp-tongued matriarch. That creates a common mix-up: Olenna is Mace’s mother, while Alerie is his wife and Margaery’s mother in the novels.