I'm not a lawyer, but this is a difficult road. You can't sue merely because somebody wrote mean or untrue things.

You have to prove that they're untrue and that they caused you harm.

In Mick's case, that harm would presumably take the form of lost income in form of work he didn't receive from other companies due to Marty's false claims. That's difficult (read: expensive) to prove in court.

Also, as others have noted, suing iD/Zenimax would also be some level of professional suicide. There are way more talented arts than there are paying artist gigs. That means game companies have hundreds or thousands of artists to choose from, and they consciously or subconsciously filter out the "risky" ones.