Widget core.comment will appear only if the page where you're placing this widget has subject and there is methods 'comments' and 'likes' for this subject:
if( !($subject instanceof Core_Model_Item_Abstract) ||
!$subject->getIdentity() ||
(!method_exists($subject, 'comments') && !method_exists($subject, 'likes')) ) {
return $this->setNoRender();
}
Hope this helps.