I've fixed it.

The only thing I did:
adding the following code to the component/com_mtree/templates/m2/page_listing.tpl.php
underneath the line:
($ this-> mt_show_review) omvatten $ this-> loadTemplate ( 'sub_reviews.tpl.php')
I've placed:
global $mainframe;
$comments = $mainframe->getCfg('absolute_path') . '/components/com_jcomments/jcomments.php';
if (file_exists($comments)) {
require_once($comments);
$link_name = $this->fields->getFieldById(1);
$link_name = $link_name->getOutput(1);
?>
<center>
<div class="mtframe">
<?php echo JComments::showComments($this->link_id, 'com_mtree', $link_name);?>
</div>
</center>
<?php
}
?>
<?php
It seems to work right. Could this also work if there're more than 1000 listings?