Japanese auto makers operated under the 'Made in Japan' stigma of the post-war years. Entering the American market was their natural move, and it wasn’t easy. They had to ensure quality fit, finish, and operation to counter this stigma. From the other hand, American auto makers seem to have been resting on their laurels, and the push to make more cars for less money harmed quality. It looked like American auto makers thought Americans liked useless trim and gadgets, while Japanese cars were more 'functional'.
Another aspect is, the innovative design, super performance and the impressive market approach all stand as proof to Japan’s supremacy at an economic level. Whoever has owned a Toyota or a Honda knows that quality is under no circumstance a product of marketing, and this is why most of the JDM automakers successfully compete with European or American auto tycoons. By any measure (defects per vehicle), failure rate per 1000 miles, initial quality, Japanese made cars are good.
Makes like LEXUS have very few initial problems, and the vehicle systems (like radio/ audio / TV, alternator, water pumps, etc.), and show a very low failure rate, over the life of the vehicle. Therefore, Japanese manufacturers clearly enjoy a premium quality over even highly rated European manufacturers like Mercedes - Benz.