Great question! If one goes by the fact that Piggy's glasses provide fire, one could assume that he is nearsighted because the glasses are powerful enough to act as a magnifying glass that provides the heat to begin fire. However, Piggy also represents the only "adult" viewpoint on the island, so some my assume that he is farsighted, in that adults can metaphorically "see farther than children" - but a closer examination reveals Golding's major premise. Ironically, it is the adults who have caused the war which has precipitated the boys' current situation. It is an adult casualty that sits atop the mountain, dead, and fearsome, even loathsome, if you ask Ralph and Roger and Jack. When the adults rescue the boys, far in the distance is a ship of war. Golding's statement, here, is that within the family of man, there is no "adult" - only older children. The boys "play" war. The adults act it out as well. The casualties are equally as devastating. So, in that sense, with Piggy representing the adult world, he is still nearsighted.
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.
Piggy was far-sighted because the children were able to start a fire with the glasses. The lense capable of starting fires are lenses made for far sighted people also known as Convex Lenses. Convex lenses are used to converge light.