This seems to be a CSS styling problem.
One of the built in css formatting of the plugin may be in conflict with your theme formatting. In this case you can disable css formattings of the plugin, and and instead of it a custom CSS can be used that fits to your theme and site.
First please try to turn on the following checkbboxes on General tab of plugin settings page:
– Disable YARPP related.css stylesheet
– Disable YARPP widget.css stylesheet
– Disable WRI styles-wri-thumbnails_woocommerce.css stylesheet