My only concern here is that rewarding people in this fashion is optimizing for the wrong thing. Rewarding for “approvals” encourages people to review the easy PRs and skip the hard ones. Rewarding for “requests for changes” encourages people to nitpick PRs for subjective stylistic things. Rewarding for merges is practically the opposite, it encourages people to merge PRs that otherwise could have benefitted from some feedback.
Hopefully, rewarding all three brings incentives back into balance.
That said, I don’t have any ideas for how to fix this while still retaining the notion of rewarding people, so maybe it’s worth doing this anyway and just keeping an eye on things to make sure this increases contribution overall instead of just redirecting existing contribution. If this produces more contribution overall it may still be a win, with easy PRs getting reviewed and merged faster (which makes everyone happy) without necessarily slowing down hard PRs.
Really hard PRs are the somewhat-controversial-tradeoff ones. These won’t be a happy story, however we gamify the easy ones.
I don’t expect such a leaderboard to redirect the type of contributions; if it can be used as a source of positive glow for those who spend effort (it looks like everyone who spends significant effort in a specific week gets into the list with the current cutoff — maybe this should be the rule of maintenance of thresholds in the future, instead of fixed top-10), it could be nice.