I disagree that AI "art" is good to use for mockup assets. For mockups and placeholders, you want something that is obviously not complete, so you can tell at a glance what needs to be replaced. Think about how many studios put out complete games with AI crap and then claim "oh that was a placeholder we forgot to change".
Meanwhile, FFXIV's placeholder assets look like this:

You literally don't need AI, you can make something in MS Paint.
(I'm 100% sure Square Enix is using AI in other places, but this proves you don't need it.)