Add Order Notes To WooCommerce Completed Order Email (add to customer-completed-order.php)
<h2><?php _e( 'Order Notes', 'woocommerce' ); ?></h2>
<?php
$args = array(
'status' => 'approve',
'post_id' => $order->id
);
$comments = get_comments($args);
foreach($comments as $comment) :
echo $comment->comment_content . '<br />';
endforeach;
?>