He might be intrigued. The fanfiction might show him a new side of the Greek mythological world that he's part of.
Hercules' reaction to reading Percy Jackson fanfiction would be complex. He has been a part of the original Greek mythology for so long. When he reads the fanfiction, he would notice the modern twists on the stories. He may be delighted to see the continued interest in his world. But if the fanfiction strays too far from what he knows to be true, like changing the motives of his twelve labors, he could be quite critical. At the same time, he might also be impressed by the creativity of the fans who write these stories.