This question was asked recently in the Formidable Slack group.
The answer is to use this snippet:

add_filter('frm_get_paged_fields', 'get_extra_form_fields', 10, 3);
  function get_extra_form_fields($fields, $form_id, $error = false) {
    if ($form_id == 25) { //change 25 to the id of the form to change
      shuffle($fields);
    }
  return $fields;
}

Change 25 to the ID of your form.