If it's only a link to a newsletter, sure. And "signing" a comment breaks a norm here. But if it's otherwise a good comment, seems like a poor use of "flag"; one of the rare instances where maybe you want to leave a short, polite note ("don't sign off with links to your newsletter, we don't sign things here").

Flagging deprives everyone else of the comment, and is especially hostile if there's already a thread sprouting from that comment.