Hi, so I did a bit of googling for you 
What I’ve found states that AI art cannot be afforded the same copyrights as an original work of art or creation and that it is allowed to use AI art commercially (so long as the platform used to make it allows public use and should be stated either on the sites main page or in their Terms of Service. )
No, I don’t think that it would be “unethical” because it’s your story and if you aren’t happy with how the current “art scene” looks, change it. Maybe take the original image and post it to your IG or socials to showcase the art made for you-- but you aren’t obligated to keep using the image.
Editing: I think it would be cool of you to credit the app or site used to make it – that way others can use them (if they are, like me, unable to draw a circle much less people lol)
Good luck in your story.