@@Merion,
Honestly, I do believe it is intentional. If it was not mirrored, the whole design would lose left-right symmetry for the body. Kinda like, if you have an arrow pointed forward (in respect to the pony's body) as a cutiemark, not having it mirrored would have it then point to the rear on the opposite side.
Yes, specific symbols look off because of this design choice, because there exist symbols, letters, and such that are only meant to be oriented a certain way, and mirroring takes something away from the reflected image. However, when you get into pictographic symbols or straight-up images... mirroring is the better way to go, using the arrow as an example. And, the vast majority of the ponies have images or symbols that take well to mirroring. So for consistency... I understand mirroring being upheld even if it takes a little away from the CM on one of the two sides.
(In the case of Octavia, even if her CM looks a little off, it's still instantly recognizable either way.)
However, with the example of Chinese symbols... That makes it all the harder. I would say you'd either make an exception at that point, or maybe not use such as a cutiemark? I mean, languages and their characters aren't established the best in the show, so I think a cutiemark like that would only exist in the realm of OCs, which is known for going a bit beyond the show's design choices sometimes.
EDIT: As for hasbro's toy line... Honestly, I think that's a quirk of manufacturing--sometimes it's more affordable and efficient to not sweat the details, and having a single symbol rather than a symbol and its mirror is a lot more mass-production-friendly.