There is more than one literary element at play in this selection from O’Connor’s great short story. As you might expect with a story this good, the elements work together.
The first literary element is foreshadowing. The grandmother is right that people aren’t nice today. She’s about to run into a serial killer/mass murderer. You can’t get more not nice than that.
The second literary element is irony. Just as the title is ironic, so is this statement. The grandmother is not a nice person, so for her to be commenting on how nice or not nice people are is deeply ironic.
A third element is understatement. There’s nice and then there’s murder, and that’s what they’re going to run into.
A fourth element is characterization. The grandmother is sort of person who makes this sort of pronouncement. She defines her world, and in doing so, defines herself.