Does the Phelps' Farm episode differ in tone and seriousness from the rest of the novel of Huckleberry Finn?

You have identified what many critics call the biggest
disappointment in this major American tour de force. Most critics agree that this last
episode of the novel featuring Huck's reuniting with Tom Sawyer and the Phelps' farm is
a real bathos or anti-climax given the central chapters that chart Tom and Jim's
adventures together by themselves and the characters they meet. They argue this because
the central chapters demonstrate the development of Huck Finn as an interesting and
valid character whose attitudes and assumptions towards social institutions of the time
reflect the criticisms and doubts that Twain himself had. Key to this development is the
relationship between Huck and Jim as a slave. Huck seems to constantly struggle to
choose between what society demands of him and what he feels instinctively is right.
This is seen for example when he determines to turn Jim in as a runaway slave because
that is what society says, but in the end he chooses to follow his own heart and does
not.

However, the Phelps' farm episode acts as a
reintroduction of the Tom Sawyer storyline, which, albeit amusing, lacks the depth and
profundity of the central chapters. Huck returns to being a mere sidekick to Tom Sawyer,
even though he has shown himself to be more than capable of making decisions for himself
when he was with Jim, and now seems a redundant character, having forgotten all the
character development that the central chapters pointed to. It appears that Huck as a
character has reverted to the Huck we were first introduced to at the outset of the
novel - a character who is more than willing to be led by the more charismatic Tom
Sawyer, in spite of the ridiculous schemes and plans he hatches, and a character who is
not willing to take the initiative himself or share any of the insights he has learned
on his journey.

There is a definite shift therefore from
the central chapters to the amusing and ludicrous "evasion" scene where Tom and Huck
embark on a series of far-fetched plans to free Jim, only to find at the end that Jim
has been freed all along.