The superstitious Jim awakens and concludes that "witches" have played a trick on him. Huck reports that as Jim tells his story, he begins to embellish the details until the witches have supposedly kidnapped him in his sleep, ridden him "all over the world, and tired him most to death, and his back was all over saddle-sores" (Twain, 16). Jim's tale so impresses the other slaves that people would come from miles around to hear it.
Clearly, through this anecdote, Huck gives us a disparaging view of Jim in particular and slaves in general. Jim, Huck feels, must be extremely gullible to conclude that if his hat turned up anywhere but on his head, it could only be attributed to the supernatural; his fellow slaves, in holding him in such awe for having experienced such an incident, are presented as equally gullible. Huck's attitude shows that he considers himself much more sophisticated than this, yet it is Huck who in Chapter Four goes to Jim to have his fortune told from a hairball.
Jim, on the other hand, is simply responding to the closeness his ancestors felt with the workings of nature, and his cultural tendency toward magical thinking. Jim is not simple-minded at all, just indoctrinated to look at things elementally. His penchant for storytelling is the prime way history and culture are transmitted in pre-literate civilizations, and his audience's close attention to his tale shows that it resonates in them with the power of myth. Huck, of course, could never be expected to grasp this concept. He does not really need to -- for by the end of the book, he will have learned to love Jim for himself. At this time Jim's cultural or even race becomes irrelevant.
The second time a trick is played on Jim, it is unintentional; Huck, whom everyone presumes to be dead, suddenly appears on Jackson's Island, where Jim is hiding out also. Jim quite naturally perceives Huck to be a ghost. Again, this is not any evidence of either simple-mindedness or childishness; ghosts and other elementals inhabit the African perception of the world.