If your theme has built-in support for displaying ratings in product grid, you can use OnVoard to sync rating and rating count metadata.
Auto sync product metafields
Go to reviews config page and turn on Sync Metafields. If this is turned on, we will update product metafields whenever a linked a review is created/updated.
Sync metafields for all products
To sync metafields for all products, go to Shopify actions page and click Sync Metafields.