No, it is not an ad hominem attack to dismiss "an argument" generated by ChatGPT. This is because only a little investigation will show that any "text" generated by ChatGPT cannot contain any arguments at all. Ever. To think otherwise is to massively misunderstand what language is and how it works. Asking this question is like asking:
Is dimissing an argument because it's machine generated an ad hominem fallacy if the argument is not an argument but a bonobo monkey?
It is a mistake of epic proportions to assign an argument to something which contains no argument. Take, for example,a set of symbols which has been generated to look like human speech.
It would be absolutely fallacious, immoral in fact, to say of such a text that it contained an argument which is either valid or sound. This is because what you would be looking at is not evidence of the intent of a speaker to influence a listener or reader's stock of assumptions in some way. This will not be immediately clear without an example.
Let's consider the following piece of supposedly machine generated text:
The man punched John. Then he punched him - harder. So they were both violent.
We cannot consider this argument to be either sound or valid. The reason is that it means nothing. It means nothing because there was nobody at the creating end of this argument who had an intention to influence a listener. There is no creator who is using a combination of context and shared knowledge to make sure that the intended interpretation of their utterance influences the listener's stock of assumptions in the intended way.
For a start, who is the referent of the word John meant to be? John Lennon, my friend John from the pub? Your uncle? We don't know. You can assign a referent to the word John if you wish. But you cannot argue the case, or make out that your assignment is correct. You might say, "Oh, but we can say it refers to some man called John". But, erm, no you can't. It could easily be retorted that it refers to an orangutang, or a tiger and there's nothing we can meaningfully say or do to counteract this.
I can hear someone say: "Well, we can nonetheless say that the argument is valid, even if we don't know who the word John refers to and whether they actually hit anyone" But, erm, no we can't. Not nearly. The reason is that we cannot know who the words he and him are meant to represent. That's because they're not meant to represent anything or anyone. And that's because there was no communicator meaning them to be interpreted in any particular way. You could interpret he to mean John, and the word him to mean the man. But, so what? Someone else can say that the assignment should be the other way round. You could argue the point, but it is a meaningless argument. You cannot clarify who the pronouns refer to. There was nobody intending them to refer to anyone. Furthermore, I have a female friend called John. And so could I easily say that in this case neither he nor him refer to John. And I could further argue that therefore the argument is not valid because I can argue that the word both refers to John and the man, and John has not been shown to be violent. But that would be ridiculous drivel too!
As nobody can definitively define any referents to any of the seeming referring expressions in the 'text' that was generated, one cannot truthfully say that it has any meaning or that it is valid or invalid or sound or not sound. To assign such a meaning to it would not only be idiotic it would be mendacious and any assessment of the text as representing a valid argument would be equally fallacious.
The words that speakers use to communicate are only meaningful because of both the communicative intention of the speaker and the successful interpretation of that intention by the listener. The language in any given utterance is normally hugely underdetermined as well as being multiply semantically and syntactically ambiguous. Language itself it not a code produced by an enigma machine for which a listener has a key. It is just an archeological trail, which given the context and shared knowledge can lead a listener to a correct interpretation of the speaker's intentions.
No speaker, no communication. No communication, no argument. You wouldn't assign validity to a camel, or soundness to a cloud. You can't assign an argument to a jumble of ChatGPT generated symbols designed to to look like an artefact of human communication.