One columnist connects the economic crisis of the late 2000s and McMansion values:

Perhaps you thought the last decade’s global economic meltdown, which crushed stock prices and McMansion values, would most hurt the wealthy. Nope. The gap between rich and poor in the U.S. expanded in the aftermath of the Great Recession. The (sarcastic) good news: America’s wealth gap expanded less than Bulgaria’s between 2010 and 2017.

Three quick thoughts:

1. McMansions are often cited as a symptom of the problems that led to the economic crisis and housing bubble of the late 2000s. The spirit of consumption in the United States with lenders providing more and more risky loans (and not recognizing the problematic loans and then selling and buying them as if they were good investments) plus decisions by consumers to purchase more and acquire debt all contributed to the larger issues. If you needed one symbol of excessive consumption from the early 2000s, commentators often go for the McMansion or the SUV.
2. While the McMansion became an important symbol, housing prices almost across the board declined precipitously. Not just McMansions were affected. And since most American single-family homes are not McMansions, it seems a bit odd to single them out here. Many Americans who would not or could not purchase McMansions felt the effect of declining property values.
3. Housing construction declined during and stayed depressed for a number of years after the economic crisis. Even during this down time, builders continued to construct McMansions. And once the economy started to pick up, more McMansions appeared. While the economic trends certainly affect how many McMansions go up, the style of home has some staying power. Even if such homes helped contribute to the economic crisis, some Americans still want to build and buy them. The value of such homes may not be the only reason people build and buy them.