The bottom line is he could drool and only say the word potato and the fact is still that we are voting for a platform and a supreme court, not a public speaker.
I think he means ethical in the way he explained it after he said that- ethical in that he respects the oath of office and doesn't use it to enrich himself.
I am sure that Cuban is politically savvy enough that there is no such thing as a truly ethical president (except maybe Jimmy Carter).
Until there is literally a smoking gun, I have no serious reason to believe that Carter is anything less than a man who has done his best to be good. He was also a human being, and I’m sure people can dig up a bad interaction if they look hard enough. Still, it doesn’t invalidate his lifetime of attempts at making a positive impact on the world.