**Scout is her nickname, it is less symbolic than descriptive**
First of all, I could not find any actual words from the author/book. There is no "official" word of why, so I sought out the definition of a scout.
According to a quick search for the google definition of "scout"
> a soldier or other person sent out ahead of a main force **so as to gather information** about the enemy's position, strength, or movements.
I bolded the part about gathering information because that is what the book is about. It follows scout as she tries to learn about the world around her. My guess is that the curious nature of Scout inspired Harper Lee to give her that name.