In most Arthurian accounts, knights have certain qualifications that allow for them to be successful in their endeavors. While some of them are qualitative, others are more quantitative. In the account, the first issue Kay the Seneschal has with Perceval is that he does not "look" like a knight. He does not ride a horse worthy of a knight and he carries shoddy weapons, both of which bring on him the ridicule of those in attendance at Arthur's court. While this gives the reader an assessment of how the knights of Arthur's court perceive the chance of Perceval's success at court, it is often not a good indicator of his actual success.
The fool and the damsel, however, see something more qualitative in Perceval, and they surmise that his success will depend on it. From their assessment, the reader can assume that Perceval will be a very successful knight. Generally, the most humble and most apparently unknightly of knights turns out to be the one most worthy of the title. The fact Perceval is so quickly dismissed by the other knights serves as the best indicator of his success.