If, when you talk about "military leadership," you include major strategic decisions such as when/whether to launch major attacks, Hitler's mistakes were very important. However, I would argue that they were less important than his political mistakes.
Hitler's major military blunder was his invasion of the USSR in June of 1941. This was something that unnecessary and overly aggressive. It turned out, of course, to be one of the biggest mistakes of the war and can certainly be seen as a major cause of Germany's defeat.
However, the war was truly lost not simply by this decision but by at least one other major mistake on Hitler's part. This second huge mistake of his was declaring war on the United States in the days after the Pearl Harbor attack. Hitler was not obligated to declare war on the US. While he had been provoked greatly by US actions, he had not previously declared war and had no need to do so simply because Japan had chosen to attack the US. There is nothing that guarantees that US public opinion would have supported declaring war on Germany in a situation where Germany had done nothing aggressive towards the US.
Thus, Hitler's lack of military skill had a great deal to do with the loss of the war, but it was not as important as political mistakes such as his opting to declare war on the US.