Buchanan is considered to have been such a bad president because he didn't think slavery was something a president could get involved in and the states would have to resolve the issue by themselves. In other words, he just sat back and did nothing while the country lurched toward civil war. There's no evidence that he was malicious, stupid or deranged, and he had excellent professional credentials: Five terms in the House, Secretary of State and Ambassador to Britain.
As far as we know: Buchanan didn't hold rallies fomenting racism or violence; he didn't threaten foreign governments or their representatives; he didn't grab women's crotches; he didn't cheat in his business dealings; he didn't lie repeatedly about facts that could be easily disproved; he didn't call the news media enemies of the state; and he wasn't under investigation for conspiring with a hostile foreign state to influence his election.
I think Buchanan's ghost can rest easy in the White House tonight.