The morality of the act of lying is debatable, depending on the context of the lie and the mindset of the person involved in the debate. Also debatable is morality of the consequences of lying; sometimes, lying can lead to positive ends, which complicates the notion that all lying is bad. It may be useful to keep this complexity in mind while analyzing the lying that takes place in Huckleberry Finn.
The character most closely associated with lying is Huck Finn himself, and sometimes, Huck's lies are for a good cause, like when he tries to protect Jim. Other times, he lies to spice up a story or to bring drama to a situation, like when he lies to get away from Pap, but even then, some might argue, Huck is lying to save himself, which is also a good cause.
The point of all this lying is sometimes self-centered, sometimes for the protection of others, and sometimes unnecessary. The absence of clear patterns around Huck's lies is worth a closer look; sometimes, perhaps the author is saying, lying is simply a way of a life if living honestly is dangerous, risky, or unhelpful to the invidividuals involved in the telling of the lie.
Perhaps some insight into all of the lying can be found when looking closely at the author of Huckleberry Finn, a humorist and satirist named Samuel Clemens, also known as Mark Twain. This writer wrote an essay titled "On the Decay of the Art of Lying" in 1882, two years before the publication of Huckleberry Finn, in 1884.
In this essay, Twain claims that "lying is universal" and that the act of lying is one that deserves "wise examination." Though his essay is humorous at many points, he concludes on a serious and valid point about lying. People tell lots of lies for lots of different reasons, as Huck Finn demonstrates, and the reason behind the human impulse to tell untruths warrants discussion. Clearly, Twain himself had a keen interest in the phenomenon of lying, and he may have written the lies into the novel Huckleberry Finn simply to indulge his own enthusiasm for the topic.