tl;dr Sarcasm is fine as long as it's helpful.
In this case, the three word "means sharp wit :)" does sort of answer the question but it's so brief, it's not very helpful. DeepYellow's comment is accurate: the StackExchange critique is off topic and was only added to make the answer long enough to submit. The critique could be easily removed by simply writing: "Quick parts means Mr. Bennet has a sharp wit." or even "Quick parts means a sharp wit.". It's submittable, but still not the finest of answers.
The FAQ does say:
Answers that do not fundamentally answer the question may be removed. This includes answers that are ... not even a partial answer to the actual question.
However, this was a partial answer. Regardless of sarcasm, the offtopic part isn't helpful and should be discouraged.
The "How to Answer" page gives guidance on what makes a great answer. Nothing specific to sarcasm, but has good advice on how to write helpful answers.
Finally, the consensus from Meta.SO is that sarcasm is generally fine, but if it's unhelpful or offensive, it's not.