If you are using the built-in Commerce Reviews, Review Reports provide information about product reviews by customer and by product.
Review Report By Customers
The Customer Reviews Report lists all customer who have submitted product reviews. The report includes the number of reviews submitted by each customer, and a link to the list of reviews.
Review Report by Product
The Product Reviews Report lists all products that have been reviewed by customers. The report includes the number of reviews and average rating, the product was last reviewed, and a link to the list of reviews for each product.