The traditional food associated with Mothering Sunday in the United Kingdom is simnel cake.
Simnel cake is a light fruitcake made with dried fruit, spices, and layers of almond paste or marzipan. Modern versions commonly have a marzipan layer in the middle and another on top, often finished with eleven marzipan balls.
Mothering Sunday falls on the fourth Sunday of Lent. Historically, domestic servants and apprentices were sometimes given the day off to return home and visit their “mother church,” the church where they had been baptized. A rich cake was practical at this point in the Lenten season, when food supplies could be running low.
Simnel cake is also strongly associated with Easter, so it is not exclusively a Mothering Sunday dish. That overlap explains why “fruitcake” can sound plausible, but fruitcake is a broad category; simnel cake is the specific traditional answer.