I am not sure whether this is a help request or a feature request, so I'm (at first) asking for help.
Often, when I downvote a question/answer and a message "Please consider adding a comment if you think this post can be improved." appears, I see there is already a comment explains why this question/answer is not good.
What should I do then? If I will ignore the pop-up, the user will not be able to understand the reason for my downvote. But duplicating the comment is not acceptable, of course. The best I can think of is upvoting the comment, but this does not clearly explains the correlation between the two.