what was the significance of Jim getting bitten as a result of Huck's practical joke?

It is not so much the snake bite that is significant rather than the superstition behind it. Huck is initially not quick to believe all Jim's superstitions. After Jim proclaims that bad luck is "a-comin', he subsequently gets the snake bite from the dead snake's friend/ mate or whatever. As the novel progresses, Huck comes to respect Jim more and more for his beliefs. Huck understands that there is more to the world than meets the eye and a coloured boy knows things that he can learn about.