This is somewhat subjective, of course, but I don't think Trump is the most incompetent president in history. James Buchanan managed to bumble his way into a civil war, something that Trump hasn't managed.
Trump's mismanagement of the coronavirus pandemic is a great example of his inability to respond to the challenges of the presidency: back in February, the White House was assuring us that the danger of a coronavirus outbreak in the U.S. was minimal. Since then, Trump has fluctuated between announcing that coronavirus isn't a real threat, to telling us that it's a terrible threat that is only getting worse; he's encouraged people to take quack cures, publicly disregarded the rules set forth by his own government, gotten into public squabbles with his health advisors, and done everything he can to shift responsibility for the pandemic onto state leaders and evade responsibility for leadership himself. All this mayhem has contributed to the U.S.'s current record-breaking rates of coronavirus infection.
These sorts of crisis have historically created a large bump in popularity for incumbents, but polling didn't show that for this president. If Trump had demonstrated strong leadership during the pandemic, rather than turning it into yet another reality-TV sideshow, he might have won this election.
