Sounds like fair use to me, unless I’m mistaken… what could be wrong with replicating a style that someone else introduced? It’s entirely possible someone else came up with the same style hundreds of years before and it just never became well known. People copy various anime styles all the time, why can’t they use a computer program to help them do it?

Clearly the argument can’t just be “because they should hire someone to do it”, heck what if that person is totally booked? It’s not taking away from them in the slightest to produce additional artwork using a similar style if they weren’t available to take the job, so that seems irrelevant.

As long as you’re not taking their work and saying it’s yours or selling it I don’t see why you can’t mimic the style.