This is a very interesting question to consider. When we think of tone we need to examine the diction or word choice and the action that a story contains. In a sense, this great story is a mix of different tones: it contains elements of bleak severity, humour and grim irony. The feuding between the grandmother and her son's family is hilarious, as is the description of the children's mother as having a face that was:
as broad and innocent as a cabbage and was tied around with a green headkerchief that had two points on the top like rabbit's ears.
Whilst smiling wryly at the mother's unfortunate fashion choice, we also see elements of irony in the tale. The grandmother uses the presence of the Misfit in the first paragraph to persuade her son into going to Tennessee rather than Florida. Of course, it is ironic that in spite of saying this it is she that leads her family into the hands of the Misfit when she takes them on a diversion. The tone of course darkens tremendously when we reach the Misfit and we understand the terrible danger the family are in.
However, these range of approaches always keep us at arms' length from the characters. Detachment is a key note of O'Connor's style. We are never allowed to become too intimate with her characters and they are all shown to us warts and all so that we are free to judge them without sympathy affecting our judgement in any way. Of course, at the end, we see these normal, average characters confront a terrible situation where the only escape for them is in death. O'Connor throughout makes stylistic choices so we can judge and assess these normal characters and see how they face this crucial understanding of their own mortality.