I have many Roman Catholic relatives, but I have never seen a picture of Jesus and Mary together where Jesus is an adult, especially one where He is on the cross with Mary at the foot of the cross. Have you ever seen such art or iconography?
I'm surprised to hear that. I see lots of Catholic art and iconography with Jesus and Mary together as adults. One very popular image are the pictures of Jesus and Mary together in the images of the Sacred Heart of Jesus and the Immaculate Heart of Mary. In all Catholic Churches that have the stations of the cross on the walls, you see images of Mary meeting Jesus as he carries his cross. Also, representations of the wedding at Cana, Mary at the foot of the cross-- basically images of them together every place where the Gospels mention them together.
Do a web search of Mary at the foot of the cross, or the wedding at Cana, or finding Jesus in the temple, and you’ll find several pictures.