The old lady next door told me that when a lovebird dies, the one left behind... it just gives up on living so it can join its mate. It was just like that.
She was deeply in love with her husband, so much that both Auron and her son believe she had simply given up living after Jecht disappeared while out on the sea on a training exercise. Tidus's mother was shocked to see her son's hatred toward his missing father, and tried to encourage Tidus to want to see Jecht again so that he could tell his father how much he hated him. She was devoted to Jecht, waiting out on their houseboat in Zanarkand for him to return. Little is known about her, except that when Jecht did not return from his training one day, she became terribly sick and died a year later.
When Tidus visits the Farplane, an image of her appears, created by the pyreflies from his memories. As Tidus initially states, his mother would not even look at him when Jecht was around, which was one of the main reasons Tidus resented his father so strongly. Tidus realizes on the Farplane that Jecht encouraged his wife to spend time with their son, and never purposefully monopolized her time at Tidus's expense.
Tidus later admits his surprise at seeing her on the Farplane, as nobody ever performed a sending for her. Yuna explains she must have accepted her death while she was still living. Tidus initially takes offense to this, although remembering something that a neighbor once told him makes him realize that Yuna is likely right.
It is enigmatic that Tidus's mother would appear at the Farplane, as she was a dream of the fayth who never traveled to the real world like her husband and son. One possible explanation is that the fayth, along with the pyreflies that inhabit the Farplane are able to conjure up memories of both dreams and real people.