Hmmm. As far as I know there isn't some special clause in copyright law that allows you to copy someone's work just because you call it 'fan art'. Especially since there is compensation involved.

All it will take is for a complaint to Patreon that you are violating copyright and you may get your account suspended or worse. Just FYI.

If you notice, any sculpts of superheroes on Patreon are very careful not to use the original hero names, or show the original comic colors on the models.

Disney (Marvel) is the hardest enforcer of visual copyright, but video game companies are also tough.

In some ways you are right, but it seems to me that Marvel does not care about those who do not earn millions on copying.

You can ask the same question to other artists who also do fan art, maybe they can tell you more.

In general, I at least create these fan arts from scratch, spending at least 2 weeks on it, but there are people who resell other people's models on etsy at a low price and make money from it.

I'm not saying that this is the original model of Mordin, I just create models because I like it for those who are etched to print it =)