Sarcasm is one kind of irony. Yes, it's typically used to expose or discredit what the writer regards as vice or folly. And if a writer does a good job, it can be both trenchant and witty -- the best, and best known, example being Swift's "Modest Proposal".
But it's still a species of irony. And you're welcome to look up that word if you don't know what it means.
OK, after reading your explanation, I don't understand why you think sarcasm doesn't describe her style of writing.
What am I missing?