Part of the significance of superstition in the novel comes in how it is used to define Jim's character. In the first third of the novel, Jim is presented as being deeply gullible, open to being persuaded by foolish ideas, and ready to believe that which is not true.
We see this in Jim's espousal of superstitions and in his responses to Huck's tricks. Later we see a different, more incisive set of traits in Jim, but initially he is symbolized by his superstitious attitudes.
Additionally, Huck is depicted as having doubts about all matters of faith, from superstition to religion. He does, however, attempt to believe or at least allow for the potential veracity of the claims of others in this regard. Yet he is naturally resistant to believing in anything beyond his own experience.
An exception to this tendency in his character is his strange belief in a false version of history (a version that he recounts to Jim with great comedic effect).
The ultimate import of Huck's relation to superstition can be seen in two ways. First, Huck's attitude is contrasted to Jim's. The two are quite opposite in matters of faith, yet they are bonded by something lasting that has no basis in religion or superstition. Second, Huck's development is related to ideas of faith and specifically to the idea of independence of thought. Huck's distance from convention and superstition marks his progress toward independence and maturity.