Are you referring to Malcy-Louise-Caroline-Frédérique Berthier de Wagram, 1832-1884?
If so this genealogy page online
https://gw.geneanet.org/orlov?lang=en&n=berthier+de+wagram&oc=0&p=malcy+louise+caroline+frederique
says that her maternal grandmother was
Anne Jeanne Rouyer and her paternal grandmother was
Maria Elisabeth Franziska de Wittelsbach-Baviere.
So if she was named after a grandmother than implies to me that Malcy was a pet form of "
Maria Elisabeth".