Golding makes it clear that Simon has been known to suffer fainting spells. This helps to encourage the other boys to question his mental alertness. Some critics have argued that Simon is an epileptic, and that he experiences seizures, which lead to his hallucination in the woods with the pig's head. Although Golding gives no evidence to this, it is certainly one way to connect his "visions" and his fainting spells.
No, Simon is not really "batty". The other boys think Simon is weird or mental because he has seizures and because he has a depth of understanding and thought that they do not. Simon understands that the beast is not a real thing; he knows that the beast is really within themselves. It's the base part of them that causes them to act like savages rather than intelligent, civilized and caring boys. Simon is the only one courageous enough to go and look for what the boys call the beast. And Simon spends a lot of time alone. He appreciates the beauty of the island. He notices things the others don't. Because of all of these differences, the boys are quick to label him as crazy.