Margaret Atwood says that in Never Let me Go, Ishiguro picks "a difficult subject; ourselves seen through a glass darkly." How far do you agree that Ishiguro has done this?

Atwood applies an interesting metaphor to her understanding of this novel. The reader does indeed look "through a glass darkly" to see a version of humanity that is disturbing, but the word glass can also be understood as a mirror rather than just something transparent, like a window. In this glass, Atwood observes a reflection that is true in its darkness, which makes the novel's themes all the more chilling.
In Never Let Me Go, Ishiguro has definitely presented a dark version of humanity, not in his presentation of the clones, but in his presentation of the society that has cultivated the clones for its own benefit. In fact, the clones are characterized as the most human of all, with a range of emotions and a depth of spirit much more complicated than the human Hailsham set. Perhaps this phenomenon reflects one theme of the novel, which suggests that humans are losing touch with the qualities that make them authentically human, so much so that artificially-made humans are more real.
No matter that Atwood's metaphor of a glass might be ambiguous. Whether she means a window or a mirror, both interpretations are apt. Never Let Me Go offers the reader both a view of the dark potential of humanity and a reflection, showing the reader what contemporary society's self-centeredness and selfishness looks like.