Sonmi-451 is Cloud Atlas's most fascinating, most iconic, and most didactic character. She talks. A lot. In fact, her sections are entirely dialogue—set up as a last interview with her Archivist before her execution. So, what does she have to say?
One of her main roles is world building. Her world, an uber-capitalistic neo-South Korea called Nea so Copros, is the first world we come to in Cloud Atlas that doesn't take place in the past or present. Sonmi has to seamlessly introduce us to the technology and customs of her society.
Basically, everything in this society is done for the express purpose of making sure that capitalism thrives—technology lets people advertise on the moon, for instance, and the government sets minimum spending limits. People aren't even called people anymore; they're called consumers, and that's their only role.
Sonmi isn't a consumer. She's a fabricant, and she has to explain that to us as well. A fabricant is a human clone made to serve consumers. In other words, she's a manufactured slave. When Timothy Cavendish jokes about customer service people that "the corporation breeds them from the same stem cell" (4.1.183), he has no idea this might come very, very true.
Not only that, but Sonmi eventually finds out that fabricants and consumers are fed the recycled body parts of dead fabricants. Soylent Green is people, right? Near the end of the novel, Adam Ewing postulates, "One fine day, a purely predatory world shall consume itself" (11.15.7). That is exactly what is happening in Sonmi's time. The consumer nature has gotten so out of control, people are eating people, and Sonmi can't stop it.
Not all fabricants feel this way. They're programmed not to. But Sonmi has ascended, meaning she's gained knowledge a lot faster than normal fabricants. In her words, "My curiosity about all things grew acute. [...] My sense of alienation grew. Amongst my sisters I alone understood our xistence's futility and drudgery" (5.1.101). Short version: she's the honest stick person in this xkcd comic.
It's not just Sonmi's sisters who are in the dark, either. All of humanity seems to be blindly following the path to destruction set for them by corpocracy. Sonmi feels alienated. When the Archivist says, "You said you envied your unthinking, untroubled sisters" (5.1.107), Sonmi responds, "That is not quite the same as wishing to be one" (5.1.108). She's not willing to give up enlightenment to become a smiling happy sheep.
So she tries to change things. Aided by Hae-Joo Im of Union, a group devoted to the Abolitionist cause (of freeing fabricants), she composes a list of Declarations, new rules for humanity to live by, urging people to be more compassionate, caring, and supportive of one another. Humanity and compassion have some issues, however, so Sonmi is branded as a traitor, her declarations rebranded as "blasphemies" (7.1.304), and she is executed.
In her short life, Sonmi at least gets to leave the confines of corpocracy, see the ocean, and appreciate it. "Why did the entire conurb not grind to a halt and give praise in the face of such ineluctable beauty?" (5.1.123), she asks. Because they're too busy buying electronics and sitting glued to screens to experience life and nature—that's why.
Sonmi's final revelation to us is that everything—her ascension, her escape, her rebellion—has been orchestrated by Unanimity. Wow, that's a downer. She's been hinting at it for a while, like when she said, "A star actress must look the part" (7.1.61). When asked why she went along with it, she says that she did because it gave her time to preach her cause (and preach she does). And, from what we learn in Zachry's time, her word got out and brought about the fall of capitalist civilization as we know it. Some people, Zachry's included, worship Sonmi as a goddess.
So is Sonmi a savior or a harbinger of destruction? And is it possible that Unanimity orchestrated the whole thing, or is that just what Sonmi wants us to think in order to turn us against them? Everyone loves a good conspiracy theory.