Precisely this. The dress code was not a secret. Once Carlsen had little chance of winning the event he decided to make it about himself with this display. No one should commend this selfish arrogance, and in no universe is it "principle" to exhibit such prima donna behaviour for attention (and to conveniently exit from a likely loss).
Magnus is a tremendous chess player. He's also, by all evidence, a massive asshole, and continuously shows boorish behaviour and terrible sportsmanship.