Ignoring your last sentence, I do find the idea of ethics on rhetorical devices peculiar. Like - if I were to use a straw man device, would that be unethical? In a sense - maybe. I'd be misleading which is a form of lying.
I certainly wouldn't say labelling an idea as "dumb" would be unethical, though. It's just not productive to having a conversation: name calling begets more name calling. It puts people on the defensive where they are not receptive to new ideas.
Straw man arguments are a fallacy, not a "rhetorical device". It's an error we're all prone to doing at some point, unintentionally. I would argue that it's only unethical if it's done intentionally—and while you can guess at someone's intention, knowing is usually impossible. So unless you can literally read minds, only the author can truly know if they were being unethical or not.
I certainly wouldn't say labelling an idea as "dumb" would be unethical, though. It's just not productive to having a conversation: name calling begets more name calling. It puts people on the defensive where they are not receptive to new ideas.