I think the sentence can be understood on its own, but I'm giving some context anyway. Editor and author talk about a character.
I know that のか is used as a sentence ending to ask a question. But I'm not sure if Author is using this word to ask a (rethorical) question in this context, because it's not at the end of the sentence, but as an enumeration. Regardless, I translated the sentence like this:
"Is it kindness? Indifference? Thickheadedness? These things are paper-thin".
My translation may be wrong, that's why I'll be thankful if anyone can help me. Also, is Author using のか to imply that the character may actually not be kind, but rather something else?