The inciting incident is the event without which the conflict and therefore, plot, of the novel would not occur. In that case, it can be argued that the day that Tom went into the Ewell house as Mayella requested and she kissed him is the inciting incident--it's the basis of the trail that the book focuses on.
However, Scout claims that "it all started the summer Dill came to us." She says that it was his idea to make Boo Radley come out--and that's what eventually happens at the end of the novel, as a result of the trial and Bob Ewell's reaction to it. The kids are more focused on making Boo Radley come out and that is their "conflict" until the book takes a more serious shift in Part Two.
When the State of Imperfection is first made explicit (in "To Kill a Mockingbird" it is the initial crime). See Kal Bashir's 510+ stage Hero's Journey at http://www.clickok.co.uk/index4.html