You can argue that countries should aim for full employment, so any unemployment by definition means they're mishandling things. But certainly increased labour supply doesn't automatically increase unemployment rate in market economies - it often has the opposite effect. Otherwise, smaller countries would tend to have low unemployment and populous countries would have high unemployment!
I am not saying that xenophobia isn't bad, but it's a bit of a leap to say that Japan is in any danger of a shortage of workers.