I voted on a comment which explained why an answer was lacking. Then I voted it down. The system prompted me to leave a comment.
After voting up the comment that I thought explained it, should I do more? It seems redundant to post a comment at that point. I assume it's just a lo p'lug but wanted to make sure.
That's an auto-prompt from the SE engine, which apparently is not smart enough to detect your comment upvote. (I guess when you vote it checks for a comment from you.) If I recall correctly, the prompt is phrased as a suggestion. In this case you already took a reasonable action, so don't worry about it.
Every now and then somebody proposes (across the network, I mean, not here specifically) that comments be required for downvotes. I'm pretty confident in predicting that that just isn't going to happen; it would make a mess and not achieve the goal anyway. If you are able and willing to leave a helpful comment, something that hasn't been said already, it's a kindness to do so, especially for newer users who are still learning how SE works. But it's entirely up to you, and if you've already upvoted what you would have otherwise said, there's no point in saying more.
While there's been some discussion about whether we should require comments for all downvotes or not (see here), I think everyone would agree that if you already upvoted a comment that explains what is wrong with the post, that you wouldn't need to leave a new comment. Doing so would just make a mess of comments, on those posts that get 10-20+ downvotes, and serves no purpose as the user who wrote that post has already been notified of what needs to be done to improve.
