Buddhism has been practiced in Japan for well over a thousand years and today remains a major influence on the spiritual lives of many people in Japan today.

Most people would indeed have known about Buddha, though the power and influence of Buddhism waxed and waned throughout the years. Sometimes Buddhism was highly favored by the common people as well as the court and sometimes, other philosophies came to dominate while Buddhism was sidelined or seen as unnecessary.

But Buddhism has been an almost ever constant player in the spiritual lives of Japanese people, as well as active players in politics, economics, and even the military history of Japan.

That being said, the average follower may or may not have known nor cared about Buddha's life history. This is because knowing about Buddha is not as central a tenet in Buddhism (varies depending on the sect) as it would be to know about Jesus' life and deeds.

Also, Buddhism has and is heavily intertwined with various other belief systems in Japan, namely Shinto. While Buddhists would have known Buddha and venerated his teachings, their relationship with Buddha is very different from the relationship Christians have with Jesus.