Witty sarcasm works best when spoken in context, as when someone says, "Now we know why some species eat their young." Out of context, the wittiness fails, but, spoken in context, the wittiness has a chance.
Timing is important, since waiting too long loses the effect, as in, "Are you always this stupid, or is today a special occasion?". Tempering sarcasm with body language can show that the remark is meant to be funny, not mean. Some cultures interpret sarcasm as insulting rather than witty, so it is advisable to know when and to whom it is acceptable to say a phrase such as, "Just because I don't care doesn't mean I don't understand."