What the widget catalog actually offers
Microsoft's own catalog of out-of-box dashboard widgets lists two Repos widgets: Pull Request, "a configurable widget that displays active PRs requested by the team, or assigned to or requested by the person logged in," and Pull Request (multiple repos), the same thing scoped to "up to 10 repositories," which the catalog also marks as being in private preview. Neither widget counts comments. Both only list currently active pull requests, so a merged or abandoned PR from last month, and every comment left on it, drops out of view once it's no longer active.
Why this is harder than counting commits
A commit at least shows up in a repository's history view with an author attached. A pull-request comment lives inside the pull request itself, and the out-of-box widget catalog above lists no widget that aggregates comment counts by person across a project. Answering "who reviewed the most this month" with the built-in dashboard widgets means opening pull requests one at a time, since those widgets stop at listing which PRs are open, not who commented on them and how much.
Where this compounds on a multi-repo project
The 10-repository cap on the multi-repo PR widget means a project with more repositories than that can't even get a single combined active-PR list from the out-of-box tooling, before comments enter the picture at all. See does Azure DevOps have a contribution graph like GitHub for the equivalent gap on the commit side, and reviewing developer activity in Azure DevOps for how to work with what the platform does provide.
Comments counted alongside commits
Commit Graph for Azure Repos counts real, non-system pull-request discussion comments per person next to their commits, across every repository in the current project, in the same daily heatmap, so this doesn't mean opening pull requests one at a time.
See Commit Graph for Azure Repos