How do I fill in this hole in my ceiling around pull light?
You call the person who did it, and say "fix it". It would have been an easier job if they moved the switch four inches inwards, and then patched in a section of crown molding.

If it has to stay there - just get a load of mud (that's the American term), and smear it into the gap. It'll look ugly, but better than the big holes you have.