Common? In the itch.io space or any indie game dev space not really from what I observed, if anything most creators here (both game devs and artists) appreciate honest feedback. Fair? Not really imo. I think most people would always appreciate honest feedback and if they're mature enough they'll see it as a way to improve, not something to be insulted of.
Unless the creator explicitly asked not to send feedback, I can't see any logical reason to block you for providing feedback. Don't think about it too much, get a bit annoyed, move on and enjoy the rest of your day :)