I guess it would be something like an “ad inhominem,” but that something was AI generated is a legitimate gripe so I hope we don’t start using that phrase.
The reason why ad hominems are a fallacy is because it fails to engage with the actual argument being made. The same seems likely to happen when arguments are dismissed because of their AI origin. While that dismissal is somewhat reasonable now, at least as a heuristic, I expect it to become increasingly less valid.