Something like this (very simplified):

  let $array := [{"foo":0,"bar":"tztz"},{"foo":"hello","bar":null},{"foo":true,"bar":"yes"}]
  let $value := for $object in $array
                return
                  let $fields := bit:fields($object)
                  let $len := bit:len($fields)
  
                  for $field at $pos in $fields
                  return if ($pos < $len) then (
                    $object=>$field || ","
                  ) else (
                    $object=>$field || "\n"
                  )
  return string-join($value,"")

will output:

  0,tztz
  hello,null
  true,yes