Right now, there isn't an option for that setting the content plugin's comments to the mobile view. We'll be overhauling those settings in the 6.2 release later this summer.
Until then, if you want the mobile view to be the default view, you'll need to edit the /components/com_jfbconnect/libraries/provider/facebook/widget/comments.php file. Around line 28, you'll see:
$tag .= $this->getField('mobile', null, 'boolean', 'false', 'data-mobile');
Change that 'false' to 'true', like:
$tag .= $this->getField('mobile', null, 'boolean', 'true', 'data-mobile');
That should get you going, but if you run into other issues or have any questions, just let me know.
Thanks,
Alex