I agree, could just be a slip up. My family, even after 1 year of being full-time, and over 2 years of them knowing about me still have some trouble with my name, even though I know they are completely supportive. If she doesn't live close to where you see each other too often then that makes it even harder for family to get into the swing of things. With my family I live about an hour away and can only visit maybe every other week, sometimes more, so they don't have the constant interaction like my roommates or my co-workers have that make it easier and faster for them to get the hang of the switch.
I would say just give her a call and casually say, "Thanks for the card, but you accidentally put my old name on the card." I'm sure she'll say it was an accident and that she just forgot.