Your JSON input should contain an array of objects consistings of name/value pairs.
It can also be a single object of name/value pairs
or a single object with a single property with an array of name/value pairs.
It can also be in JSONLines/MongoDb format with each JSON record on separate lines.
You can also force double quotes around each field value or it will be determined for you. The output CSV header row is optional.
See also CSV to JSON and CSV to GeoJSON
Plus Convert JSON to XML, XML to JSON, JSON Lint, JSON Formatter and Analyze JSON Paths at ConvertJSON.com