Hi,
and thank you for writing in!

To see the Best Seller badge you should enable the option in YITH Plugins -> Best Sellers -> Show “Best Seller” badge.

If it’s enabled and you cannot see the badge, your theme probably doesn’t use the standard WooCommerce hooks and functions: for this reason you don’t see any badge on products.
YITH WooCommerce Best Sellers uses the following hooks to add the badges on product images:

• woocommerce_product_thumbnails
• woocommerce_before_shop_loop_item_title

In this case you should contact your theme developers and ask them to add/use these missing hooks in their theme.

Best Regards
YITH Support