Well, first let's talk about what myopic means literally. Myopia is a scientific term that means near-sighted. A person with myopia can clearly see things close to them, but has a hard time seeing things in the distance.
Once you have this picture, it isn't hard to see how one can turn this term into a figurative or metaphorical slam at someone who only sees or thinks as far as they need to. A myopic person (figuratively) is one who only wants to see what they believe, and won't open up their eyes (mind) to see points of view that are "out there" or different from their own.
Tying this into Lord of the Flies, Piggy may physically be the only one on the island who is myopic (since he is the only one with specs), however it is Jack who is the metaphorical myopic, because all he can see is hunting and power. He refuses to think of the needs of the group and concentrate on rescue.