Okay, a search of the text of "The Godfather Returns," by Winegardner, includes a Cast of Characters that identifies Mama as Carmela. It looks like the book was published in 2004, and searching on here it looks like no one used the name until 2005.
I've never read any of the sequel or prequels, but it looks like that could be the source.
Edit: very strange they'd pick the same name as Mrs. Soprano, if the Winegardner book is, indeed, the source.
It's a lot older than that. If you look online you'll see Mama referred to as Carmela in the screenplay and shooting script of GF and GFII.
My guess is that Puzo named her Carmela when he was developing the character. Even though it never made it into the novel or movie, it was her name in the script and, therefore, known to the cast and crew and eventually made its way into public view.
