Scout can be considered a reliable narrator because as a child she has no agenda, and does not sugarcoat or ignore the facts.
Scout is often considered an unreliable narrator because she is a child, and as a child she does not know or understand everything that is going on, and because an older Scout is telling the story from memory. While this is true, we should not paint Scout as the unreliable narrator so broadly. As a citizen of Maycomb, she knows more about the town and its happenings than most outsiders. She is also a very perceptive and precocious little girl.
Scout’s ability to parse the facts and come to the truth is demonstrated by how she talks about Maycomb.
There was no hurry, for there was nowhere to go, nothing to buy and no money to buy it with, nothing to see outside the boundaries of Maycomb County. But it was a time of vague optimism for some of the people: Maycomb County had recently been told that it had nothing to fear but fear itself. (Ch. 1)
The reader can tell from this example that Scout is more aware than the typical youngster, and is capable of reliably narrating the story. As a child, she does not bend the facts to her will. She simply reports them. Sometimes she does not understand them completely, but no adult would either. This makes Scout the perfect narrator for a story about how adults let emotion and tradition get in the way of common sense.