I often see him on the top of the worst president lists. Is it true? Well lets see....
He did a rather poor job with the Bleeding Kansas incident. He had good intentions trying to stop the fighting between the pro-slavery and anti-slavery governments there. But his actions were meddled with political tactics and a struggle for power. His support split quite a bit even among the Democratic Party.
One of the worst things to happen during his time as President was the Panic of 1857. The cause was linked to the overconsumption of European goods, the fall of many Railroad companies, and bad credit of many banks. Apparently recovery didn't take too long, but while waiting many people especially up north suffered quite a bit.
During his reign there was also the Utah War. Where he replaced the governor with a non-Mormon. Despite shitty entry in the conflict, it did allow a lot of peace at the end with pardons given out to everyone.
Near the end of his presidency, the Democratic Party was split in two. The divide led the Republicans to gain most of the control during the later years.
Near the end of his term, he oversaw the foreshadowing of the Civil War with the prediction that Abraham Lincoln would be elected President of the United States. He really did nothing to make the states stay within the Union.
Was Buchanan a bad president? Well yeah pretty much. Worst president ever? Hmmm... that's a tough question. But I would have to say not yet.