Fair use is determined on a per case basis so on the strict legal side there's no "generally", however the majority of original artists out there wouldn't pursue fan art creators because that's shooting themselves in the foot, in a way fan art is free advertisement and unless the fan creator is messing with the business of the copyright holder you very rarely will find yourself at the point where a court will have to determine if your work is fair use or not, this as long as you don't mess with big guys like Disney.

So the fact are that :

At he base level fan art is creating a derivative version of a copyrighted work, which is bad.

It can go from bad to grey if a court determines your work falls under fair use (note that selling fan art will work against you here, that's commercial gain).

Original content creators very rarely go after fan art creators because they gain benefit from it and it's generally not worth the legal cost, so in the real world you're quite safe selling fan art or all the other derivative stuff being sold on places etsy.

Now this is about the legal, on the in-game advertising side I don't think they have made any specific statement whether they allow advertising or not (it is RMT even if no gil is involved, see also people selling runs for real money), but because of what I said before of fan art benefiting them I highly doubt they would go after artists advertising, but if you want a peace of mind you can send a GM call and ask for their stance on advertising commissions in-game.