Although I cannot find a direct quote that mentions whether Piggy is either far or nearsighted, I believe one can strongly argue that the character is indeed nearsighted. (Being very nearsighted myself, I am quite familiar!)
First, Piggy's glasses are very thick, something usually associated with nearsightedness. He cannot see much of anything except blurs when he removes them.
But more important is understanding the nearsightedness on a symbolic level. Piggy can see what is right in front of him when he has his glasses on (both literally and figuratively). Again, this speaks to his "nearsightedness."
Piggy and the glasses, when he has them on, represent intelligence and the ability to see clearly.
Finally, the breaking of Piggy's glasses represents the "progressive decay of rational influence on the island." Someone needed to see things "close up." The boys killed the only person who could do this.