I'm no monarchist, by any means. But I'm not sure 'tacky' is the word I'd go with.
Tacky, to me, implies cheap, false and/or overly ostentatious. The tiara on the right is definitely tacky, because it's about 3x bigger than her face, and is almost certainly made of rhinestones/glass/plastic. If it were genuine, I'd say 'gaudy' or 'garish' would be better descriptions.
I wouldn't say that the Queen's tiara (in this photo) crosses that line, though. I mean, she is the queen. Surely, if anyone can wear something like that un-ironically, it's her. Besides, it's reasonably-proportioned, and fits a particular context. Now, if she popped it on for a run to the shops, or to muck out the barn, I'd say it's a bit odd. (Though, at her age, I'd say screw it; she's allowed to be 'eccentric.')
I mean, I know what you're getting at. It all does seem a bit much. I want to call it pretentious, except that, strictly speaking, I don't think it's possible for a royal to be pretentious. They're the ones that set the bar that pretentious people strive for.
But, at any rate, I don't think tacky is quite the right description.