If they do this every four years, then every survey requires them to rank a president who's either been in office only a year or, in some cases, would be a year into his second term. How you can even give an assessment of a first-year president's "greatness" is beyond me, but I guess anyone can grab a number out of his head to represent a guy he just plain can't stand.
Typically the presidents at the bottom either got us into an awful war, screwed up the economy or found themselves mired in a serious scandal. Trump hasn't done any of that, but he keeps tweeting, so someone has to be #45: