A world where everybody gets what they deserve is a just world - is that so bad?

I'd say that depends very much on who decides who deserves what.

Justice is inherently malleable and subjective. It's a thing we humans (as a society) decide to do to one another. As far as I can tell, it tends to lag the scales we're operating at.

I'm assuming it would happen automatically - i.e. people would be judged by all-knowing and impartial external parties (angels, aliens etc.) - otherwise of course it cannot happen, i.e. of course people alone can never implement true justice.

As for defining what is just - this is obviously a huge subject studied by philosophers - but I'm assuming that, if we had these all-knowing judges, they could just assess (by scanning thoughts of people) how hard everybody is trying to do good to others. And this way, we would be living in a just world. To repeat my question - would that be so bad?

Yes.