Nosto for Magento supports ratings and reviews powered by Yotpo. In order to begin leveraging Yotpo ratings and reviews in Nosto, you'll need to install Yotpo's extension for Magento.
Once you've installed the Yotpo for Magento extension, you'll need to follow their guide on setting up "rich snippets" in Magento.
You can switch the rating and reviews metadata provider to be Yotpo by navigating to the configuration page and toggling Ratings source to Use Yotpo under the Ratings & Reviews section.
Note: the Use Yotpo option is only visible once Yotpo's extension for Magento is installed.
Once you've enabled the rating and reviews metadata, you should be able to see the new product metadata collected by Nosto.