You generally cannot copyright a style.
If it spits out entire chunks of pre-existing works, that's an entirely different story; but what it seems to do is (via the learning and subsequent diffusion process) receive an input like "Wonder Woman on a hill" and (to falsely anthropomorphize a giant math puzzle) say "I know what a Wonder Woman looks like, and I know that 'correct pictures' have some certain ratios of straight lines and angles and tend to use some particular color triplets, so I'm biasing the thing that matches to my 'Wonder Woman' shape structure with those lines, angles, and colors." The result is a picture Hollie Mengert has never drawn, which an observer could assume is done by her because the style is so spot-on.
And aping an artist's style is not illegal for humans and we have no law to make it illegal for machines. Should it be illegal is an interesting question, but it will require new law to make it so.