What is Lee trying to express in To Kill a Mockingbird when Dill says "I ain't cynical. Telling the truth's not cynical. Is it?"

This is a short but revealing conversation that Dill has
with Jem and Aunt Alexandra (in Chapter 22). It comes in response to Dill's revelation
that his Aunt Rachel is a closet drinker who

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"... drinks a pint for breakfast every
morning--"

one of the few
references to alcohol mentioned in the story. Aunt Rachel is angry because Dill was
missing (joining Jem and Scout in the courtroom for the trial), but Dill claims to
have

"... told
her till I was blue in the face where I was
goin'--"

Dill's remark comes
as a surprise to Aunt Alexandra who, as a newcomer to Maycomb, is not privy to all of
her neighbors' secrets. It is the only mention of this in the story, so we can assume
that Rachel keeps her habit to herself. Alexandra calls Dill a "cynic" because she
assumes that the story is a lie or an exaggeration; Dill believes his aunt was probably
too inebriated to remember that he had already told her where he was going. Of course,
Dill is not the most credible character in the story, either, and Alexandra is probably
right to question his statement. However, Dill knows what he saw, so if what he says is
true, his words are not particularly cynical. In any case, it shows that Dill is slowly
growing up and seeing things in a new light; as a "cynic," he has lost his naive view of
the things around him.