Only if you accept a potentially unlimited number of CSV files/sheets. Many forms of data aren't really easily normalizable to a limited number of flat tables without losing information.
Open it in any decent plaintext editor/viewer, it'll likely have support for 'prettyprinting' json, and it'll be readable.
If you want to 'do stuff' with the data, then JSON is a very (IMHO most, but your mileage may vary) reasonable format; unless that data really is just a single flat table, it would be hard to blame them from picking this format.
I suppose that some service could handle it. But then there's another level of trust and GDPR compliance.