This is not supported by this plugin.
In the background YARPP plugin works, it searches similar content for all pages based on its static content.
In case of cart page, YARPP can find related contents according to the static part of cart page, but the important part of cart is its dynamic content: the products that are in the cart actually. This is not considered by YARPP in case of cart page.
Of course it is possible to make this functionality for a specific webstore application, but it can be another plugin.