"Mayor of 'X'" is an expression sometimes used in politics to describe someone whose authority has a much narrower base than it theoretically should.
Defining the term is much easier through example, though. The most frequently use of this expression in current media is the description of Hamid Karzai, the President of Afghanistan, who due to the unstable nature of his country and his narrow power base, has little power outside of the capital, and is therefore called "The Mayor of Kabul".
I have also heard the term used, in variants, to describe domestic politics in the United States of America. For example, Ted Kulongoski, the former governor of Oregon, was sometimes called "The Governor of Portland", due to the fact that he won election mostly through a large margin in the liberal city of Portland, rather than wide support throughout the state.
I don't know the origins of the term, and like many terms, I doubt it was coined deliberately, but rather just rose up from the ether that is language. It is also not used extremely widely, but it does pop up now and then, when it is appropriate.
192 for BQ12