It would be the same as copying someone's writing and not correctly citing it, marking it down and pretending to be the author of it = plagiarism. Would be considered deception and fraud.
Not clearly mentioning that the content was copied from someone else, either from another human or from an AI prompt, is a fraud.
Generally, by default, any reader considers that the content they are reading was created by a human, and when commenting and communicating, they presume that they speak to a human.
Readers generally feel deceived and cheated when they find out that the response or post was generated by AI without telling them.