What is wrong here? Issue with post_id and meta_value

You’re close, but not quite. Try it like this:

function jb_applicant() {
   $custom_fields = get_post_custom(2171);
   $op = '';
   foreach ( $custom_fields as $key => $value ) {
      $op .= $key . " => " . $value . "<br />";
   }
   return $op;
}
add_shortcode('applicant', 'jb_applicant');