"Most nice people are merely signaling their niceness."

That is ridiculous. What you are calling "signalling" is the actual niceness. People that have a reputation for being nice have been engaging in nice behavior. Behavior is ultimately the only thing that matters since we can't look into someone's brain to see if they are truly nice or good or whatever. Someone that always issues you a cheery greeting, asks if you need some help, remembers your birthday, and brings donuts to work is legitimately nice. Their reaction to some meaningless made up problem is in itself meaningless. People who behave like a nice person are superior to those jagoffs that don't.