I will say that the USN's massive increase in size is impressive - it's genuinely very difficult to rapidly expand a navy to the extent that the USN did, and going down blind alleys like the Casco class is the sort of thing you expect when a nation without much experience in such matters is doing things very quickly under war emergency conditions.
It's just that that doesn't make it the strongest navy in the world. It might well mean that it had the most ships on its navy list and the most guns claimed afloat, but as I once found when looking into it this is because the navy list includes ships that were merely planned and in some cases never started, let alone finished.
What the USN actually did during the ACW is that it proved itself able to rise to the challenge of expanding by a manpower factor of ~ten (from 8,000 to about 84,000) in four years without falling apart. This is, in and of itself, impressive.
What it did not do during the ACW is build the most powerful navy in the world in those four years. It takes longer than that to grow capabilities and it takes sustained funding over a longer period of time than that to build a base of reserve manpower and ships; the Royal Navy in 1862 had a peacetime strength in manpower terms about 10% less than the USN's wartime maximum manpower (78,000), and in addition had 22,400 men in some form of pension, reserve or volunteer establishment which could be called into the navy in the event of war. And purpose-built steam warships, already built and waiting in reserve, for all of them to sail in.
That's the difference between the USN (a force which at full wartime strength consisted mostly of suitable civilian ships and gunboats built quickly for the war, manned by men who had been trained since the start of hostilities) and the RN (a force which could without any further enlistment of men not already accounted for field a fleet larger than the USN in manpower and consisting entirely of purpose-built warships, including literally dozens of the capital ships which take years to build).