I think that's a great point. It's an easy way out to frame a discussion like this. "McMansions exist because people are tacky and have bad taste".
I think it's more interesting to look at it from the other perspective. Why is it such a popular design?
Maybe the quaintness of pre-war/post-war housing did not keep up with the needs of modern life. Maybe we need more space, and more privacy from the ever expanding sprawl of suburbia to feel safe, and sheltered (in a mental sense). The cheesy facade could just be a simple tool to keep the building's quick-and-dirty plaster walls, within the scope of the suburban vernacular.
I actually grew up in a McMansion. I always thought it was a great house. We had a pool, and two concrete lion statues guarding the front door. Thinking back on it makes me cringe. How could my parent's buy a house like that? It oozed with tackiness.
But it got the job done.
It was large, had enough room for the whole family, and was a physical manifestation of their hard work.
When neighbors stopped talking to each other and only focused their attention on their own families, it became harder to flaunt wealth. Maybe the McMansions are just a clearer, and louder, cultural identifier, compared to things like gold watches and fine clothing?