too sensitive to a rider

Question
hello ma'am, what does "THIS HORSE IS TOO SENSITIVE TO A RIDER" mean?

Answer
I am not sure in  what context you found this statement, so I am doing a bit of guessing here.

It sounds as though they are commenting on the horse being sensitive, and therefore needing a good rider. A good rider has an independent seat, so that his hands and legs always agree, so that he can be consistent in her aids to the horse.

I would think that this horse would be reactive if he were kicked or yanked on, but no one should do that to a horse anyway.

I have seen horses in the States who have been ridden roughly and abusively, just go into a "learned helplessness" mode, where they barely react even to bad or rough handling.

It is the rider's responsibility to learn to ride to the best of his ability.