Income is more concentrated today compared to the post-war period, but I don't think that makes the mean meaningless---it depends upon how much weight you put on a dollar held by different parts of the distribution. That being said, I think you'd be hard pressed to show that any part of the income distribution is worse-off in real terms than they were in 1950.

Incidentally, we only know about these changes in the income distribution because empirically oriented economists have been collecting and analyzing this data for many years.