Charlotte was never a top rider. Valentine is in a completely different league.
Even when Charlotte was riding regularely she used to enter 1.10 or 1.20 competitions. I'm not sure if she even rode a 1.30 ever. That's amateur level. A good amateur level, but very far away from the top riders.
If you keep in mind that she has the means to have the best trainer and the best horses, well... it is kind of obvious that she is not specially talented.
I'm not saying she is bad, mind you, she is a good amateur, but nothing else. The only reason she competed in top level events like the Monaco Horsejumping is because grandpa introduced amateurish pony level competions so that his grand niece and friends could have some fun.
While Charlotte has been retired from the horse riding world the Roziers have been compiting all around with her old horses like Folka and they have been riding top level competitions with them and getting good results, which means that the horses were very good and they were been a little bit wasted with Charlotte.
Charlotte can't be compared with Valentine, and even less with the such as Athina Onassis, princess Anna, or Zara Phillips who really rides top level.
I'm not criticizing her, mind you. No one is under the obligation to be a brilliant horse rider!!! She is an interesting cultured young woman who happens lo like horse riding and who has a very good level for an amateur. Nothing wrong with it.