Absolutely. Usually there's no problem in doing both (hence, I guess, the OP)

But commenting without downvoting skews the ordering algorithms, and of course if someone else has already made a similar enough comment that you wouldn't be adding materially to the conversation, adding a similar comment just makes the post look busy an therefore, potentially, interesting.

In my other comment here, I suggested a solution to the duplication problem - you can downvote only if you either create a reply or upvote an existing reply (as a sign of agreement).

That would be a nice feature. Or much like a "report" button on some platforms, if clicking downvote meant you had to say why you had disliked, and it gives you the usual moderation options, all the other comments that it could auto-upvote, or a free-text field that automatically creates a comment... that would probably be the least-friction way of dealing with it.

That would be great! I agree with parent comments that sometimes downvoting without commenting serves no purpose.

You got a downvoted comment with no replies. Now, why was it downvoted? You can reread it and see if you were rude/offensive, but besides that...