what does "you're no picnic" mean??

You’re not easy to get along with.

It's likely a response to someone complaining about how difficult someone is. The difficult person might reply, "You're no picnic, either," meaning that the original speak is also difficult.

I've never heard anyone say you're no picnic but I have heard doing something or going somewhere is no picnic, meaning it wont be easy and/or fun. So it probably means the same thing when said about a person.

I was wondering the same thing along time ago