Cats, like people, have unique personalities. Some cats are outgoing and seemingly fearless (I picture many orange tabbies that I have known). Others are shy, easily frightened, and introverted. Either extreme, as well as anything in the middle, is completely normal. There’s nothing wrong with being shy.
It sounds like Minx had a rough life before you adopted her. Most pets with that sort of background are shy when they are adopted. Once they have a good home, many become more confident over time.
Also, remember that Minx is still a kitten. Just like shy children can grow into confident, outgoing adults, many shy kittens become more sociable and less skittish as they mature.
The best way to help Minx become more confident is to provide a stable home environment with plenty of love. Fortunately, most kittens are pretty lovable, so hopefully that task won’t be too much of a challenge.
It is most likely that, over time, Minx will grow out of her shyness. However, there is a small chance that no matter what you do she will continue to be shy and easily startled. If that happens, it does not mean that there’s anything wrong with her.
On a different subject, one thing you mentioned does trouble me slightly. It is Minx’s reluctance to play. This reluctance could be due to her shyness, but sometimes failure to play can be a sign of a medical problem. If you haven’t already taken her to the vet for a basic checkup, I recommend that you do so to make sure that her health is good.