Saying he’s “moral” doesn’t really mean anything unless you define what his morals are.
Carter didn’t do anything to stop Roe v Wade for example.
Carter is a true believer in Christianity, more so than any of the others in my lifetime. Since being in office, he has dedicated his life to charity, rather than focusing solely on accruing wealth.
This doesn’t mean he was a good or effective politician, but in my mind, he seemed like the most honest man who has held that office. The fact that I disagree with him on policies is a different issue.