HN guidelines would prefer that you don't comment on downvoting, as it makes for boring reading. If you don't have anything substantive to add (such as, if "others have pointed out" your thoughts already), just vote and move on. :)
Although the words "I'm downvoting you" aren't useful, I would argue that saying why you're downvoting someone is really important. Helps prevent the feeling of shooting the messenger.
https://news.ycombinator.com/newsguidelines.html