I think the idea with McMansions is that they are just tacky. Poorly aped styles. Columns that don’t do anything and are proportioned wrong for the load they are intended to look like they carry. Complex roofs that do nothing useful but “look fancy”. 100% style over substance, but with style that snooty people look down on.

I imagine that McMansions are generally about as energy efficient (per square foot) as other contemporary homes, though.