It's actually quite sound: JSON Resume wants to standardize CV fields to improve compatibility between tools, ease conversion, ease theming and such.
It's a joke because it sounds good on paper but the execution is awful. I tried it out and the resulting CVs are look completely unprofessional, except they took three times as much effort.
I got all my data in there, although I found the schema to be very limiting. Then I went through 50-70 themes and none of them actually looked good when exported as a PDF or something you can email to someone else. Note to theme creators: 30px padding everywhere does not look good and makes things harder to read.
I could have customized a theme to fit what I wanted, but the schema was garbage. At that point, I might as well toss the schema and generator and implement my own resume template using something like Underscore, but it was a lot simpler to use Word.
I don't really understand what part of it constitutes a "joke".