It's not a high bar for Trump to clear, but the worst Post-Presidency probably belongs to John Tyler. Never elected, he became President when William Henry Harrison died in 1841. President Tyler couldn't even secure his own party's nomination in 1844 and left after one term.
President Tyler returned to his plantation and resumed farming. Tyler owned slaves, and had even brought some to the White House. Tyler's neighbours appointed him as the County Overseer of Roads to mock him, but the ex-President worked hard at the job.
Tyler returned to national politics in 1860. He raised a company of white militia to protect his enslaved property from northern abolitionists after John Brown's raid. He then got elected to the Virginia Secession Convention and headed the committee to join the Confederacy
Tyler personally signed the Ordinance of Secession and was named to the Provisional Confederate Congress. He was then elected to the Confederate House of Representatives, but died before he could take his seat.
President John Tyler's funeral was the first state funeral held by the Confederate States of America. President Jefferson Davis had the body lay in state in Richmond, the new Confederate capital, then led a procession at the funeral.
Tyler is the only US President to not have had his casket draped with the American flag at his funeral. He was instead was draped with Confederate Stars and Bars.