That comment was probably deemed to be unsubstantive - you replied "no" to a lot of stuff without explaining why you were disagreeing with it. (I would have just downvoted that comment, not flagged it - I only flag comments for serious policy violations like incivility, spam, etc.)

I would downvote a one-word comment like "No." because this class of comment does nothing to further discussion.

If it were "No, and here's why" then I'd upvote it instead.