The default product thumbnail image is 300px. It works well if the shop page has 4 columns. In some cases, when you use 3 and 2 columns, the thumbnail blur because the width increases but the thumbnail size is still the same and smaller.
All you need is increasing the default thumbnail size by going to Customize -> WooCommerce -> Product Image
Then you must regenerate the thumbnail to a new size. WooCommerce supports this features in WooCommerce -> Status -> Tools