As an American, yes I believe many (though not all by any stretch of the imagination) are insular. But having lived both sides the pond, I wouldn't say they are any more or less insular than the British or the Irish.

People tend to view the world through the lens of their homeplace first. If you have travelled far and wide, you are likely not very insular at all. If you are deeply rooted to your home and have travelled little, you are likely insular, no matter where in the world you are from.

I think what you are really getting at is are Americans as arrogant about their insularity as the British perceive them to be?

My answer is, I think British and American arrogance about their nationalisms are pretty equal.