Well, I saw above someone referring to British newscasters also wearing sleeveless dresses in the dead of winter so maybe, jacqui24, you're right, it's not an American thing anymore...! However, only in the US, I've had difficulties explaining salespersons the difference between wool and cotton, explaining friends what a "closet's change-of-season" is, etc.! But maybe it has to do more with the fact that now we're all air-conditioned in our cars, in stores, etc.!
The dress itself is probably not meant to be tailored (a good view here
https://shop.nordstrom.com/s/jason-..._-1&siteId=TnL5HPStwNw-EbAMuf..FiSJ_oyYqHs5iQ) but it's odd. I'd love to see a view of Miss Markle standing but I haven't found one. The dress (without epaulettes) is not in stores so it looks like she may have borrowed it directly from Wu's showroom which would explain why (always in my opinion...!) it does not fit too well.
Although I rarely like their choices, I really don't think Beatrice and Eugenie choose their outfits to make themselves "look silly" like you're implying. I think they just don't have a good sense of style but think they look good. Well, I have seen MM wearing better outfits but I still don't like her style. And for me, style also means, hair, posture, and jewelry choices. It will be interesting to see if and how she evolves.