Although you can go to https://jsoncanvas.org/ itself and see an example rendering, you cannot see the exact data that created it - I think, although you can sort of guess since the element names are stuff like node.
I sort of doubt this is the best data structure for representing this kind of thing. Maybe I'm wrong though but I would think I would go for something like https://github.com/jsongraph/json-graph-specification which strikes me as closer to graphml which I have some experience with, and maybe give it ability to embed videos etc. (which for all I know someone already has)
This is all an initial feeling though, like hmm, no I think it's wrong, and maybe I am just not seeing why this would be better than another solution.
An "infinite" canvas without some notion of recursion such as viewports feels incomplete.
Obsidian's implementation of JSON Canvas supports this.
The upside is that it does not leave the most important aspect open to interpretation.
But it prevents this from being text-only at the point of creation:
You'll most likely need some programmatic environment to create non-trivial diagrams.
But then the question is: Why not just an SVG instead?