Hi,
I personnally don't see that the thumbnail of that image is particularily blury on your link.
The thumbnail is smaller and a bit compressed, so of course there is a bit less details to it. But it looks quite reasonnable to me.
You could change the code:
$quality = array(
'jpg' => 95,
'png' => 9
);
to:
$quality = array(
'jpg' => 100,
'png' => 9
);
in the file administrator/components/com_hikashop/helper/image.php in order to not have compression on the thumbnails generated by HikaShop. But I don't think a 5% compression quality change will do much.
If you really want to have better thumbnails, then you would need to use a better algorithm to resize the images. And that's not possible in PHP as there is only one. So you would have to resize the images into thumbnails yourself with an image where you would have a lot of control on that (like photoshop) and then replace the thumbnails in the /images/com_hikashop/upload/thumbnails/ folders of your website via FTP. This could be an harduous process, but I don't see any alternative if the thumbnails generated via PHP are not good enough for your needs.