I don't care what tools someone uses, as long as they know and agree with what they're handing me - and agree that what they're handing me is what they want to play. If someone has a few ideas but isn't good at connecting them and wants to use ChatGPT to do that work, its fine, as long as they've actually read what comes out and are interested in playing that character.

What would be bad would be if a player assumed that I was trying to assign them homework or get them to prove their commitment to the campaign or something, generated a bunch of stuff, didn't bother to read it closely, and handed it to me as 'here, assignment done'. If e.g. I then make reference to something in that backstory and they're like 'huh?' or RP totally differently than their backstory suggested for their motivations or relationships, then that's worse than if they just didn't hand me anything.

I don't need or want a book, I don't need or want prose. If the LLM gives them inspiration, great! If running a few ideas through an LLM makes it more readable and organized, that's useful. If it turns what could have been a bullet point summary into a page of filler, well, I'd rather they just not bother but its not like it will offend me.