We know the hair has to be a big deal, because the very first words of the novel are, "The boy with fair hair lowered himself down" (1.1). And it just keeps growing. Here's a small sampling of what that "idiot hair" (8) gets up to: Ralph's eyes "yearned beneath the fringe of hair" (6); "He would like to have a pair of scissors and cut this hair—he flung the mass back—cut this filthy hair right back to half an inch (7.2); "His hair was full of dirt and tapped like the tendrils of a creeper" (12.1). Trust us, there's more.
In particular, Ralph is constantly playing with that hair. He "pushes" his hair off of his face twelve times in the novel. Doesn't sound like that many? It is. If Golding takes the trouble to describe a character does something twelve times—in twelve chapters—believe us, it's meaningful. And what it means is savagery. Ralph's growing hair is a symbol for the gradual breakdown of law and order. It's a reminder of just how far he is from civilization.
Golding isn't exactly saying, "Cut your hair, you dirty hippy," but he's close.