Looking back over the post-World War II period, all the presidents seem deficient on various grounds and none stands out as exceptional. Major debacles and scandals exist: Kennedy had the Bay of Pigs; Johnson the Vietnam War; Nixon added Watergate to his handling of Vietnam; Ford had the Nixon pardon; Carter had the Iran debacle (and both Ford and Carter suffered through stagflation); Reagan had Lebanon, Iran/Contra, and giant deficits; Bush had Iran/Contra and stopped short of victory against Iraq; Clinton had the Monica Lewinski scandal; Bush 43 had the Iraq invasion;giant deficits, and the subprime collapse, Obama seemed to under-react to almost all issues, and then there is Trump. Apparently, the last president truly admired by most was Eisenhower. There also was plenty of good news, with various president deserving credit: civil rights, Medicare and Medicaid (Johnson), moon landing (various, but it happened under Nixon), Environmental Protection Agency and opening to China (Nixon), a (temporary) balanced budget (under Clinton), expansion of healthcare under Obama. I'm not seeing a previous president that has done (1) nothing positive, (2) only important stuff or (3) one that was consistently incompetent. It's not impossible to be one of these. Long-time prime minister of Singapore Lee Kuan Yew was about as good as it gets as a leader; multiple examples of all bad abound: think Saddam Hussein of Iraq or Muammar Gaddafi of Libya.
Some presidents have been better than others, but, it seems to me, none break out of the mediocre category. Lets call that "somewhat unsatisfactory." Some gave great speeches, like Kennedy, Reagan or Obama. Some had a progressive agenda, like Johnson; conservatives often cut programs, like Reagan or Bush. At least three presidents (Nixon, Bush 43, and Trump) seemed to believe in the "imperial presidency," basically the president being above the law with vast powers both domestically and globally--and pursued operations that likely violated the law. This worked out badly for the US under both Nixon and Bush. Republicans likely view Reagan as rising above the mediocre label, while Democrats probably would say the same about Obama. Overall, perhaps we would have to go back to Kennedy or Eisenhower to get general agreement putting them above mediocre (I could argue either way).
In The Fifth Risk, Michael Lewis describes the complexity of the federal government and why it's necessary to brief the incoming appointees (some 4,000 of these, 1,200 of whom need Senate confirmation) in detail. A presidential transition team is required by federal law and Chris Christi headed up the one for Trump (who thought it was a terrible idea and did not want to spend money on it--he later accused Christi of "stealing my money"). They identified people qualified for the appointed positions and did background checks. Trump fired Christi and the team on day one and tossed the work in the trash (actually it was Steve Bannon who actually did the trash can thing). He slowly picked people for the various positions, most of whom seemed unqualified and often hostile to the departments they headed (apparently considering the employees all stupid and lazy; also purging anyone identified favoring Obama). "The federal government provided services that the private sector couldn't or wouldn't do: medical care for veterans, air traffic control, national highways, food safety guidelines. ... the basic role of government is to keep us safe. It manages a portfolio of risks. ... A bad transition took this entire portfolio of catastrophic risks--and made all the bad things more likely to happen and the good things less likely to happen" (p. 25). Despite a good transition effort by Bush, Obama did not have a great transition and appointed some poor managers (note the rollout of HealthCare.gov).
Obama created a great transition experience across all departments and agencies for 2016, with thousands of people spending up to a year to prepare the 4,000 new political appointees. This is important because the previous administration's appointees quickly disappear (and forbidden from initiating contact with their replacements). On day one (Lewis used the example of the Energy Department) Obama's team was waiting and no one from team Trump showed up. Rather than a large team spending thousands of hours, new Energy Secretary Rick Perry eventually showed up to talk to outgoing secretary Moniz for a few minutes but with little interest in seemingly anything (Perry is the guy that couldn't remember that Energy was the third department he wanted to eliminate, "oops"). As stated by Lewis: "We don't want you to help us understand; we want to find out who you are and punish you." ...They mainly ran around the building insulting people" (p. 41). The Energy Department handles all nuclear weapons and attempts to keep nukes around the world away from terrorists. Mistakes here are catastrophic. Mr. oops, Rick Perry, replacednuclear physicist Ernest Moniz; before Moniz was Nobel Prize winner in physics Steven Chu.
Lewis basically took the overviews meant for team Trump, which is mainly what the book is about. The federal government does important work by dedicated professionals, which is mainly obscure to most of us (my experience is similar, mainly with the Securities and Exchange Commission). Each new administration means thousands of new political appointments and a vast learning curve even for those with considerable experience and dedication. Lewis documents that some important administrators, without any guidance from team Trump (and presumably no paycheck) just walked out the door without having spoken with their replacements. The long-term harm of the Trump approach is unknown, but likely substantial. Other sources also back this up and point to many high level permanent employees quitting in disgust and frustration (especially true in the State Department). Tillerson was called the worst secretary of state ever, only to be replaced by Pompeo, seemingly even worse.
Major department involved in global conflicts (especially the military, intelligence community and State Department) have conflicting goals and agendas. Books about Vietnam, 9/11, Afghanistan and so on make this point again and again. The idea that intelligent, hard working specialists would use billions of dollars (plus massive military might) to undue the work of other intelligent specialists seems especially repellent to me. It requires a president and leadership team(s) to be aware of these conflicts and get them to work together that seems absolutely required (but seldom seems to have happened). I suspect Eisenhower was the last president not to get rolled by the military and intelligence agencies. Think, for example of Kennedy at the Bay of Pigs or Obama's "surge" in Afghanistan. One result has been the decline of the State Department as an effective deterrent to conflicts around the world (see, for example, Farrow's War on Peace). Within the military is the rivalry across branches; the rivalry between the CIA and FBI probably contributed to the success of 9/11. What is the chance a president can get all players to a single set of mutually agreed upon objectives? Poor, aka mediocrity.
The president has a daily briefing by intelligence, a group with a unique agenda (or perhaps multiple agendas). In other words, the president gets bombarded with a specific perspective continuously. I assume the presenters believe they are objective (perhaps a heroic assumption), but who knows if this approach is in the long-term public interest. The roles of the military and intelligence are expanding, the role of diplomacy declining.
A general issue is the lack of honesty and deceptiveness in politics. A candidate for president (or just about any office) has to appeal to either Democrats or Republicans (and then the general voting public) with specific agendas. No one will be truthful about, say, Social Security [which is, benefits will have to be cut and revenue raised]. Presently, Republicans seem masters of deception, Democrats relatively incompetent.
One point worth mentioning is the classified nature of much of the federal government, which means news is often not reported. Unknown serious events may have happened with no outsiders knowing about it. The job of president seems next to impossible in the best of times and I have no warm feelings for any of the post-war presidents. Trump has the potential to break out of the mediocre range, but only in the lower direction. Here's rooting for mediocrity (and no serious crises) in the current administration.