The sentence in question simply uses a figurative meaning of the word "ripple." The sentence does use figurative language, but it does not use any specific literary device.
Old Mrs. Radley died that winter, but her death caused hardly a ripple—the neighborhood seldom saw her, except when she watered her cannas.
When Scout reports that Mrs. Radley's passing barely causes a "ripple" in the town, she is implying that when the lady died, the town of Maycomb was like a shallow body of water whose surface was just barely disturbed by the news of the lady's death.
If you refer to the Oxford English Dictionary, the first listed figurative use of the noun "ripple" is "the mildly unsettling effects of some event." This is the meaning that Scout, the narrator, intends when she describes Mrs. Radley's death as just barely causing a "ripple" among her neighbors. So, in this sentence from Chapter 8, there is no specific type of figurative speech in use aside from the figurative meaning of the word "ripple."
You might ask, "Wait, isn't this a simile or a metaphor, since Scout is comparing the town to a body of water and Mrs. Radley's death to something that strikes the surface of the water and barely causes a ripple?" No. If the text had actually stated something like "Mrs. Radley's death was like a pebble that hardly rippled the waters of Maycomb," or "Mrs. Radley's death was a pebble that hardly rippled the waters of Maycomb," then we could identify a simile or a metaphor, respectively. But the narrator simply uses the figurative meaning of "ripple," with no simile or metaphor, so we readers are left to imagine for ourselves any implied comparisons. That is the effect that figurative language in general has: it uses words in a way that departs from their literal meanings.