$comments = $wpdb->get_results("SELECT * FROM {$wpdb->comments[$wp_id]} WHERE comment_post_ID = '$id' AND comment_approved = '1' ORDER BY comment_date");
を、
$comments = $wpdb->get_results("SELECT * FROM {$wpdb->comments[$wp_id]} WHERE comment_post_ID = '$id' AND comment_approved = '1' ORDER BY comment_date DESC");
<?php $comments = $wpdb->get_results("SELECT * FROM {$wpdb->comments[$wp_id]} WHERE comment_post_ID = '$id' AND comment_approved = '1' ORDER BY comment_date DESC"); ?>