I've always considered McMansions to be more of style than an actual building type, usually it has to do with a mismatching of architectural styles, almost within the vein of "what a poor person thinks a rich persons house would have". Some of the diagram like posts on McMansion hell are a good example here is one, https://mcmansionhell.com/post/699110995935805440/a-fine-sel...

Also I think many of what is considered a mcmansion to be mass produced, that is if wikipedia is to be believed.