...well, at least as a security question.
This is not a new thing, and is the reason that most places allow you to define security questions other than that. With the prevalence of information available online, it really is not much of a stretch for a would-be attacker to determine; if you are connected to your mother via a social network like Facebook, where having a public maiden name is encouraged to help you be found, it becomes trivial.
If you still have “mother's maiden name” as a security question somewhere, there's no need to panic. There are two simple options. You can see if there is another question you can use in its place, and just use that one. If you can't (or don't want to) do that,
lie make something up; there is no requirement that the answer is accurate, only that you can match the answer when challenged. What you should not do is demand that your mother remove that information from her profile. This is unnecessary, as the information has already been released; plus, “security through obscurity” is of limited benefit.
Just a quick tip to help keep you secure online…