Why would you post your rejection publicly? You can bet that this can be used against you in several ways. (copycats, sharks, people seeing 'rejected by 'x' as a negative'.
Better to learn as much from your rejection as you can, swallow and prove them wrong.
There are lots of times when openness is a good thing, being rejected by YC may or may not be such a time, it depends on a lot of factors and if you open up make sure you've thought through the consequences of doing so.
Typically in the VC world, when a VC passes on an investment they keep very quiet about it to avoid tainting the seeker of funding.
You've got a point about VCs. (Hollywood works in the exact same way) Some industries act like a high school hive-mind popularity contest that runs on social proof over logic. Perhaps it works better for them to think emotionally like customers and users do. However, I've gotten 3 job offers when I posted my failures on HN. I've gotten no job offers when I posted my successes. Overall as hackers/entreps we understand that "failure = successful trying" so sharing failures on HN has it benefit.
If you're feeling really dejected, just one positive comment can make you feel a lot better. But it's certainly at your discretion; don't post if you feel it would be harmful.
