No. Nash was a mathematician, Einstein was a physicists. Math and physics are entirely different. Mathematics can be used to describe physical phenomenon, but it has a number of concepts that don't exist in the physical world. Things like discontinuities, points, singularities, and even lines and planes, that don't physically exist. These concepts are necessary and useful in solving problems in physics, but don't necessarily always explain what really happens in the physical world. Something that works in mathematics, doesn't necessarily work in physics.
Very well said norwaypinesavage. It is hard not to speculate just how much Nash knew about physics as I'm sure he had some training. But as you say the two are different fields. Also, many times a breakthrough in our understanding of math and sciences do not happen in a vacuum. If Nash was on to something, others around him would or his students would be working in tandem.
I had a college level algebra professor (a PhD who really couldn't teach maths) tell me that she was thinking about going back to the books to get a degree in quantum physics. At the time I thought to myself that this individual probably couldn't give a good high school definition of quantum physics. This lady I'm convinced thinks that she could solve all of the problems that the Ford Motor Company has by looking at company statistics. No marketing skills or business background, just consult the data.