The comment.tpl.php is located @<drupal-root>/modules/comment/comment.tpl.php. If you want to customize it for different content type. Copy it to your theme folder and rename it to
For example, the machine name of Article is article. So the file name should be comment–node-article.tpl.php.
- StackOverflow – Drupal 7 comment.tpl.php overwrite for specific node is not working
- Drupal API – comment.tpl.php