Is there a hook I can use to alter the way comments are rendered?
You can use hook_comment_view():


  The comment is being viewed. This hook can be used to add additional data to the comment before theming.


e.g.

function hook_comment_view($comment, $view_mode, $langcode) {
  // how old is the comment
  $comment-&gt;time_ago = time() - $comment-&gt;changed;
}


Or use a standard theme preprocess hook

function MYTHEME_preprocess_comment(&amp;$vars) {
  // Make changes to $vars that will be available in comment.tpl.php
}


You can also override the standard comment.tpl.php file in your theme to have full control over the markup.