I've heard the term "McMansion" since at least the 90s. Used in my family's neck of the woods (Fairfield County, CT) to refer to new construction that is over-sized relative to the lot, and often designed in a horrid Faux-Colonial style. Sometimes built to replace legitimate Colonials from the 18th century, for good measure.
More generally used to describe new homes built for people with far more money than taste.
Edit to add: Also used to describe spec house developments in wealthier towns. Those were for people who had more money than taste and didn't care that their McMansion was like a Levittown house on steroids, with each floorplan in the neighborhood exactly the same.