A year ago, I was frustrated (and surprised) to see one can't do code reviews with Jupyter Notebooks. GitHub diffs for notebooks JSON are super messy.
I set out to build ReviewNB[1], code review tool for Jupyter Notebooks. Turns out a lot of other people had this exact problem. One can see visual diffs & write review comments on notebook cell. Currently only works with GitHub though.
If you want to diff locally (before committing changes), you might like nbdime[2].