Think devs are pretty averse to dressing Sims like Royals. Think there was a tiara in TS2 that I never touched in my entire time playing all of those games.. Not fussed. If anything it seems like it would have come with Glamour Stuff but you never know, there was concept art developed for a TS4 wedding pack back during the polls for the community pack (Later Laundry Day).. Sounds like something a tiara could potentially come with, if they ever released a wedding themed pack.
I'd rather not see a million townies walking around in their usual rags with tiaras on though.