Not flip flops!
In UK Regionals Manx/BRAL/Flymaybe uniform was the norm for sim. Though rightly or wrongly (not clearly defined in those days) I positioned a few empty flights in non uniform, and would advise my FO similarly. Who wants a long positioning journey afterwards (eg taxi or pax flight back to base) in uniform?
In the corporate world (Legacy) mainly non uniform in the sim. And for real flights (if we were empty) I would always dress casual. Shorts/Jean's, T shirt and sandals (not flip flops - could slip on the pedals) if the climate suited. Though always had a semblance of uniform available if plan changed (as it often did). Mid flight change into uniform and vice versa often the case, (flight attendant told not to look if she didn't want to see me in a state of undress!). EU, USA, Asia, Africa airports had no problems with crew in non uniform. UK probably frowned the most.
If only doing airline flights in uniform then maybe sim in uniform too. But if EVER flying for real in casual clothes, then should be able to fly (and sim) in whatever feels comfortable. And makes it quicker to get to the hotel bar on arrival 😀