There is an app called jellyfish which I think gives some info about the PRs.

another way is to use the API for bitbucket cloud and create a script/program to retrieve the information: https://developer.atlassian.com/server/bitbucket/rest/v805/api-group-pull-requests/#api-api-latest-projects-projectkey-repos-repositoryslug-pull-requests-pullrequestid-blocker-comments-get