The main thing that I remembered of Carter post- Presidency was him "observing" elections in 3rd world countries and certifying the elections. He was President before I was really aware of politics so I have little memory of that time.
I am not sure if I would call him the worst President or not. I would need to see a better list of "accomplishments" to judge by. My view of the job the Bush's did keeps falling lower and lower.
As a person, he seems a lot more decent than a lot of the politicians in recent years.