u/Red_Galiray has posted here a good answer as to why Buchanan is considered a bad President.

After his actions in the Dredd Scott case and in Kansas encouraged the South to be even more intransigent, it is indeed hard to imagine what he could have done that would have stopped the onset of the war. However, once Lincoln had been elected, Buchanan watched the South prepare for war but did nothing to prepare the North. He could have moved to secure the Harper's Ferry Arsenal, or at least empty it. He could have supplied and fortified the Charleston forts, including Ft Sumter. And he could have fired Secretary of War John Floyd, who was a Southern sympathizer, before Floyd moved arms into the South. But Buchanan was himself a Southern sympathizer: he viewed the war as being caused by the North's unwillingness to accept slavery. Being a Northerner who sympathized with the South was how he had managed to be elected President in the first place, like his predecessor, Franklin Pierce. And so it's not surprising he was reluctant to ready the North for the conflict.