I believe in American exceptionalism but I happen to believe it is due to our Constitution and our Bill of Rights. Not the genetic superiority of the people.
There's an interesting subquestion to the issue. America was founded, and raised up, in large part by people who fled persecution for their beliefs. Many founders came here with next to nothing, all had to start from the dirt up, and some rose to substantial heights in education and earthly prosperity in a short time. The early arrivals had to rely on their own families, initiative, and wits to learn because there were no institutions in place. Many of our first arrivals and their descendants were achievers, self-motivated, often quite intelligent, and in general, filled with faith in God to help them. They knew the blessing of relying on Him. Their descendants kept pushing forward to build a strong educational system and other societal supports that rewarded diligence. From this crop came exceptional, diligent people and from that brain trust came our founding documents. We still have our fair share of lazies but I believe this nation stands in a place to be envied for the qualities mentioned above, particulary qualities of character and faith in so many of our founders.