A trailing /s is the convention. Failing that, perhaps less subtlety? Text is poor at conveying tone.

I thought it was pretty obvious and the italics made it more so.

Don't use sarcasm on the internet (or at least not on HN), it usually ends with a bunch of responses which didn't pick up on the fact that it was sarcasm which lowers the quality of the conversation. Remember also that many HN readers are not native english speakers and are unlikely to pick up on the true meaning of a comment if you're not really clear. Even suffixes like /s are not clear IMHO (at least, I didn't know what it meant until I saw the other comment here suggesting it and I AM a native english speaker).

Jokes and such are frowned upon on HN and many people probably see sarcasm the same way, so you risk getting downvoted too.

In order to avoid adding noise (in the form of responses from people who didn't realise you weren't serious), I would suggest to refrain from sarcastic comments.