Phabricator has supported reviewing Jupyter notebook diffs for some time. The feature isn't perfect, but it's a lot better than trying to review raw JSON.
The 2020 Week 19 (Mid May) release fixes two of the roughest edges with it:
- You can now click to expand nearby context in a Jupyter notebook diff.
- Inline comments on a Jupyter notebook no longer include "context" from the raw JSON source file in email.
Here's context expansion in action: