The significance of Jack's mask is that it becomes a facade behind which he can throw off all shackles of civilized behaviour and instead behave like a truly savage hunter. It is in chapter four that he first paints on his hunting mask and we see the beginning of his slide into barbarity.
Jack evidently delights in his transformation to a savage. When he first saw his reflection he, "...looked in astonishment, no longer at himself but at an awesome stranger...." (p. 80). Soon after he shows a complete surrender to primitive behaviour - "He began to dance and his laughter became a bloodthirsty snarling" (p.80).
It is also significant that later Jack's hunters all use the mask of paint. It not only signifies that they are the hunting party, but that they no longer feel the rules of the group as led by Ralph apply to them. When Ralph's group is later reduced to a pitiful number and condition, Samneric suggest that they too should use paint, presumably to feel more powerful and more connected to the group who have the strength. Significantly, Ralph shouts down the idea as he doesn't want his group to look like 'savages'.