I agree with you that from Gucci PR point of view her presence is certainly a succes. But I don't think that Charlotte thinks the same way.
By reading her interviews it is obvious that she clearly cares for the sport and wants to be a succes in it. She doesn't want to be just a pretty face and a body to hang clothes on. She wants to do things, and create and achieve something with her horses. She is quite clear about that (i must find time to scan the self service interview). She doesn't want to be a passive figure but do things with her life.
Also, if she just wanted to be Gucci's face she could ride the 1'15m, along with other celebrities, she would get better results and appear in the trophy photos and spend much less money on horses. And the general public wouldn't know the difference. They would think she is a champion because she appears receiving trophies on pictures.
So, no doubt Charlotte riding career is an image and PR succes for Gucci. But being a Gucci face is not Charlotte's aim by itself. Owning 7 horses and travelling the international circuit is awfully expensive. She uses Gucci to pay for her horses.
What I mean is that even if the press and public don't care for her sport ambitions, for Charlotte, Gucci's promotion is not a goal, but a means. She works for Gucci to be able to pay for what she really cares about: her riding career. So, no matter how pretty she looks on Gucci clothes, is she doesn't succeed on horse the result of the Gucci Masters is a personal failure for her.