No. America has never had a mono culture. It was founded specifically to avoid it. One of the primary goals was the enabling of religious conclaves within the country. The Amish, the Mennonites, Catholic focused Virginia. We have always had pockets of their own culture. And that’s just the stuff we did deliberately. Because we’re a post Industrial Revolution country there’s also the city country divide. And then as westward expansion more pockets started forming. The common culture myth of America has been wrong for as long as there’s been America.
Very very late to this party, but the only times I’ve ever seen “monoculture” referred to in regards to the US is in terms of entertainment.
If there ever were institutions that bound Americans, it was the three television networks and the major movie studios.
Otherwise, I don’t know what else there is. I grew up in a town of 3,000 people. There was a creek that ran through the center of town. On one side were Catholics. On the other were Protestants. And until the mid 70s, you needed a very good reason to cross that line. If an all-white small town can have that type of division, a multi-racial, multi-ethnic nation of several hundred million has no chance of being united in any meaningful way.