Miki recognizes the sarcasm, but pretends not to understand it because people's reactions to having their sarcasm taken seriously is funny.
Miki does understand sarcasm, but doesn't respond to it in such a way that would suggest she understood it.
Double Subverted: But she frequently misses sarcasm anyways; she only claims to ignore it for laughs to hide this fact.
Parodied: Natalie says something she thinks is ridiculous in a sarcastic tone of voice. Miki knows it's actually true and thinks Natalie has figured out her secret.
Zig Zagged: Miki does recognize when Natalie is being sarcastic, but only sometimes. There are times where she takes what Natalie says literally, or responds with sarcasm herself. Sometimes, she deliberately pretends that she doesn't understand just to get on Natalie's nerves (or if she is the subject of sarcasm, which would overlap with Insult Backfire).
Averted: Miki understands sarcasm as well as you would expect.
Enforced: "We need Miki to be the odd girl in the group. How about we create an inability to understand sarcasm?"
Lampshaded: "You're using a rather strange tone, and saying things that are clearly false. Why?"
Miki's mother raised her not to understand sarcasm because she didn't want to deal with a snippy Bratty Half-Pint.