Mitrovic's red card was just about as unprofessional as you can get. There had been a couple of minutes since the VAR referral for the penalty, so it wasn't just a spontaneous reaction to the decision. Also, it was a VAR pen, not really the refs decision, along with the fact that William's red card was automatic after the pen decision, again not really something the ref had a choice over. Only logic to it was 'but you should have given me one in the first half' which is ... unprofessional. The other thing is the ref backpedalled after he'd given the card, if he's stood his ground Mitrovic would have had his head in his face and facing a very long ban.
Having said all that, I hope he doesn't get what Di Cannio got when he pushed the ref over (11 games), which was much worse.