It does appear that the sentence style exhibited in the first part of "To Kill a Mockingbird" differs from the style in the second part. And, debate continues on whether Truman Capote, Harper Lee's childhood friend, assisted her in composing this novel. So, doubt has been created by the fact that Harper Lee never wrote another novel after receiving the Pulitzer Prize for this one; she eschewed interviews and was extremely taciturn when confronted with one. In addition, Capote seemed very disgruntled after Lee's winning of the Pulitzer Prize and their friendship cooled thereafter.
Inspection of the narration does reveal a sentence structure that is more sophisticated in the second part of the novel. For instance, while figures of speech are employed in both sections, the second part reveals sentence construction of more complexity. And, there is a familiarity to the sentence structure of works of Capote such as the novella, The Grass Harp.
Here is a passage from Part One, Chapter 3:
It was no use. I unlatched the back door and held it while he crept down the steps. It must have been two o'clock. The moon was setting and the lattice-work shadows were fading into fuzzy nothingness. Jem's white shirt-tail dipped and bobbed liek a small ghost dancing away to escape the coming morning. A faint breeze stirred and cooled the sweat running down my sides.
Now, here is a passage from Part Two, Chapter 16
A wagonload of unusually stern-faced citizens appeared. When they pointed to Miss Maudie Atknison's yard, ablaze with summer flowers, Miss Maudie herself came out on the porch. There was an odd thing about Miss Maudie--on her porch she was too far away for us to see her features clearly, but we could always catch her mood by the way she stood. She was now standing arms akimbo, her shoulders drooping a little, her head cocked to one side, her glasses winking in the sunlight. We knew she wore a grin of the uttermost wickedness.
There is a rhythm to the sentences of Part Two that is absent in the first part. Not every sentence begins with the subject, they are longer with more phrases, and the language is more poetic. Still, the difference is arguably subtle.