He's not completely irrelevant. His total incompetence led to the Ayatollah seizing control of Iran from the Shah and taking the hostages, which marked the beginning of the tidal wave of whacko Islamism. The relevance of this came into particularly sharp focus a few months ago.
Where Carter failed, was to NOT bomb the Iranians for what they did to our embassy. Bomb them back to the stone age. THAT is the Carter legacy: 9/11, and the absolute lack of fear the Muslims have for the US.