Some interesting comments here. I have flown for several airlines (lifestyle has always trumped 'loyalty'!) with differing ideas on uniforms in the sim: three that were non-uniform, one that insisted on uniform, one that went from uniform to non-uniform and one that went non-uniform to uniform. I have never seen uniform make the slightest difference to any other pilot's performance. The 'sloppiest' dressed that I have ever seen anyone in the sim were the trainers when that airline went uniform to non-uniform, yet they were respected by all the pilots because they were good trainers (which they were) not because of anything that they wore. I have seen good and bad trainers in both uniform and non-uniform sims, but the worst were in uniform sims and the best were in non-uniform sims.
As for any argument about 'realism' in the sim, any non-normal situations that I have had to deal with in the real world have all been far more relaxed than in the sim giving more time to focus on the situation rather than what "she/he in the back" wants to see. With one situation over two decades ago we even had a brief joke before starting our approach, glancing back to the area behind the captain's seat where the trainer would be sitting in a sim "OK you can reset it all now"! What clothes we had been wearing at the time would have had no influence on the conduct of the situations nor the outcome.
"Was there ever an incident caused by a pilot not wearing his tie? -- No!".