Some facts: America is one of the last nations that still use Fahrenheit scale to measure the temperature. The scale was the primary temperature standard in English-speaking countries until the 1960s. In the late 1960s and 1970s, almost all countries switched to the Celcius scale, but not the United States. Fahrenheit is used also in the Bahamas, Belize, the Cayman Islands, and Palau.[source] And that's how it looks like on a map: (Fahrenheit: green)
So yes, I think Americans are stubborn. Are they the most stubborn nation in the world? I don't think so - I think that for instance Islamic nations are much more stubborn. Most of the Asian nations are probably also more stubborn. From the western world, France comes to mind. But generally I believe Yes, more than an average nation, especially comparing with western countries.