Up until mid-90's, the Air NZ rule was civvies, sim instructors didn't have uniforms.
At that time, the HF people (Ex-CRM) stumbled across "enclothed cognition" (Adam and Galinsky)
"Enclothed cognition" seemed to have its genesis in an third-rate social science journal (the journal didn't have a particularly big score for citing): its central theme, IIRC, was that crew-members perform better when they are dressed according to role.
I thought it BS then and while still not convinced, allow that my STEM preferences and education may be clouding my view.
Sim instructors got uniforms at about the same time.