Everyone can be mean at times, just not the Pisces. Isn’t Pisces known for being the kind and selfless people? I guess, they could appear to be mean to you because of some of the reasons:
They are straightforward people when alone. Pisces commonly lock themselves in their own room for an hour, not because there are anything wrong with them. Socializing for the whole day probably put them on an autopilot of the sort. They need time to recharge and gather their thoughts then feel productive afterwards. So if you did find a mean Pisces and want to be left alone, he really just might want to be alone.
Pisces are introverted by blood. They often actively tries to meet and get new people, but then wouldn’t know it when those people already switch and ends up with real actual friends. It becomes really difficult for the Pisces to be around or hanging out with people that he doesn’t feel that close with. So it might come off to you as being closed off or fake. In reality, it’s just their inborn capacity for curiosity to find out about people’s lives and stories. Let the Pisces be themselves, excuse them for the many questions that might instead sound blunt and affronting.
They could also be really oversensitive. If one your Pisces friend has been quiet lately, you might accidently hurt them with your words by accident. Be careful with what you said, although be honest with your feeling as well, since Pisces can sense the wariness in your emotion.
You might want to know more about Pisces: http://zodiac-signs-astrology.com/zodiac-signs/pisces.htm