Good question. US defamation law is fairly weak[0], but all the usual exceptions that make it weak wouldn't apply. e.g. "truth is an absolute defense against defamation" doesn't apply because AI generated or photoshopped nonconsenual porn is fake. I'm not a lawyer, but I think a defamation case would at least survive a motion to dismiss.

[0] Which, to be clear, is a good thing. Strong defamation law is a generalized censorship primitive.