Mrs. Cake used to be named Chiffon Swirl>>36829474
Huh, actually now that I look at it, you're right. They never say her name in flashbacks, so it could've been Cup Cake before she married Carrot, and just coincidentally ended up with the same last name. Well, it's a remote possibility, at least.
And they do say Buttercup is her nickname, but it really seems like grasping at straws, since Apple Bloom, Applejack and Big Mac all had no idea that wasn't her real name. I can't imagine she didn't change it to her nickname after the marriage. All the ponies who changed their names are still the wives, and it's just hard to conclude that Pear Butter wouldn't have changed her name regardless of the feud, as per the marital tradition.