The first is not really an idiom. It means that he jokes around a lot.
The second is an idiom. It means that she is trustworthy. She will handle a task without making mistakes.
Interested in Language
I got difficulty in understanding idiom:
1. He was a bit of joker.
2. She was a safe pair of hands.
Welcome to the forum, Lovin.
What has your thread title got to do with anything?
A good title would have been A bit of a joker.