Pull request may have many iterations. Reviewers write comments and pull request's creator has to fix/settle them. Until comments are not fixed reviewer has nothing to do with pull request.
A problem is that there aren't any info in pull request list (except of last updated time) which can help to recognize pull requests that can be reviewed and pull requests that are returned for fixing.
Reviewer can decline pull request and creator will create a new pull request after fixing all comments. But in this case all pull request's activities will be lost and reviewer must check all changes again instead of checking only new ones. That is why I avoid declining pull request if it is not completed or needed some changes.
For my opinion, the best solution would be a new status of pull request like - in progress or incomplete or something else. With this status reviewer would be able to mark a pull request and filter off it.
The simplest solution is changing additional info in pull reqeust list
from: last updated 2 hours ago
to: last updated 2 hours ago by John Smith
But this solution has many pros