It's a new dress she likely had made for this event months ago, and I don't think she would have paired that color dress with a ruby tiara normally. So I do think Catherine was asked to wear this tiara somewhat last-minute.I guarantee she didn't pick this one herself. This has King Charles written all over it. So in that case, I think it's perfectly fine she wore it.
I think there may have been an intention to wear an updo, but it wouldn't have worked without a lot of structural fake hair pieces added in. As much as I personally love Swedish tiara hair, that is not the norm for British royals, and I do think if Catherine showed up last night with massive fake hair pieces she would have been ridiculed mercilessly online. The internet has a creepy obsession with her post-cancer appearance as it is- a giant updo of fake hair would have just fed that.
Personally, I love tiara surprises, and am happy to have seen this one back in action. Given the size issues with modern hair, I'm sad to think we may not see it again for a long time.