One lovely case of sarcasm is "praising" someone's (epic) fail with "Otsukaresama" or "Gokurosan", things you usually say to appreciate someone's (hard) work.
But unless you don't understand the whole story and been following the whole conversation so you can be 100% sure you are able and qualified to be sarcastic about other people's escapades (as sarcasm can be offending, no matter what culture you're hanging with), just be serious.
I understand that being able to use several kinds of speech including sarcasm, can show your language/conversation skill, but unless you're good enough to sound natural, it'll look ridiculous. Imagine the wannabe rapper trying to fidget like the real thing, ending up jerking around like a spastic.