Actually this is not the case. By the time it was introduced massively, a significant part of the air war was over. After escorting bombers to their targets, the P51s were then detached to perform the other thing they were really good at - ground attack - shooting up trains, troops and equipment.
In fact, one of the conclusions of the Strategic Bombing Survey is that the collapse of the German army might have come a lot faster had the Army Air Corps been focused more on ground support and less on strategic bombing.