The P-51 in real WWII conditions was an excellent fighter. Real conditions, however, include many things that don't matter in the Main Arena: range, speed at altitude, maintainability, and production, for example.
I think that the jist of the first post, though, is correct. I think that the popularity and recognition of the P-51 (the "glamor girl" of US fighters) has a lot to do with it being the most-prevalent US fighter for air-to-air combat over Europe in the later stages of the war. That and the fact that it is a beautiful-looking plane.
There are two US glamor planes of WWII, planes that even the general (non-WWII-enthusiast) US population tends to recognize: the P-51 and the B-17. They are both excellent and beautiful planes. However, as a couple of other examples, the P-47 fought more than the P-51 during peak power of the Luftwaffe, and the B-24 carried more bombs, was faster, had a greater range, and was produced in larger numbers than the B-17. The P-47 -- and similarly the P-38 -- and B-24 seem not to get their share of notice, speaking comparatively of US aircraft.