Parzival is the "Fool" in von Eschebach's tale because of his over-protective mother. Growing up sheltered from the knowledge of knights by his mother, who lost two sons to knightly combat, Parzival thinks that the prince in the shining armor that he encounters is God. He shows his foolishness by asking about the armor that the knights have on:

The boy said another thing that made them laugh. "Oh Sir God, what can you be? There are so many little rings tied on you, up here and down there!" And the lad's hand was tugging at all the iron he could find on the prince. "My mother's young ladies carry their rings on ribbons," he said, examining the knight's armor, "but theirs do not lie so tight together .... I can't pick any of it off". (72)
Parzival is also the fool in a more serious sense. He fails to ask about the grail or the bleeding lance when he first sees them at the Grail Castle and his folly causes the suffering of the wounded lord and the land to continue.

Selections from Wolfram von Eschenbach's Parzival. Translated by A.T. Hatto