You could change this in JComments template:
1. Open file /components/com_jcomments/tpl/default/tpl_comment.php
2. Find code:
<span class="vote-<?php echo $class; ?>"><?php echo $value; ?></span>
and replace with:
<span class="vote-good"><?php echo $comment->isgood != 0 ? '+' . $comment->isgood : ''; ?></span>/<span class="vote-poor"><?php echo $comment->ispoor != 0 ? '-' . $comment->ispoor : ''; ?></span>