First, I would not pay much attention to those presidential rankings, they compare too many apples to too many oranges. But the ones they usually place at the bottom ( Pierce, Buchanan, Johnson) did a bad job just before or after the Civil War, and the one at the top, Abe Lincoln, did a good job. Grant's case is more complicated.

Grant had a job previous to being Prez. Grant become an effective general in the Civil War, and should get some credit for winning it. He did so with McClellan's better trained and organized army, and with the manufacturing power of the northern states behind him, but his taking of Vicksburg is an example of how he could focus on what was important and keep after it, and not lose his nerve. That was what worked, in the end.

But when Grant became president it soon became obvious that he was not a politician, did not understand how his new job was in some basic ways much different from his old one. He assumed, like a military officer, that he would give total loyalty to his staff and they would have complete loyalty to him. But it's one thing to ask people to put their lives at risk for you, another thing to hire them for a desk job. He did not realize that proximity to the President created opportunities for corruption, and resisted believing that his personal secretary, cabinet, or even his in-laws could be making money from it. He was not corrupt, but he should have been able to suspect and detect corruption.

He did come into office with some attractive ideas. He wanted to implement Reconstruction, have better relations with Native Nations: but, when he found neither was popular with the voters, he was not politician enough to campaign for them, despite being , still, one of the most admired people in the US. He did create the first Civil Service Commission: but would not override the political interests of his own party to get rid of political patronage. The real creation of a non-partisan civil service would have to wait until Chester Arthur.

Although Ron Chernow has written a typically Chernovian-positive bio of him, I think William McFeeley's earlier assessment of him is more accurate. Grant was a competent general and admirable man who was promoted beyond his training, at a time when a truly masterful politician with a clear vision was needed. Because of that, he was in a position to implement Reconstruction, manage relations with Native Nations, really reform the Civil Service, but didn't. As he was surprised by scandals around him, he became more and more just a time-serving bureaucrat, doing what he was advised to do by people he considered more expert. Perhaps the saddest example of the shrinkage of his ideals was when he came back from his world tour and tried for a third term as president. He didn't think he had to actually lay out a platform, a list of goals, reforms: he thought he could just say he wanted his old job back. Even for a medium-sized business, that attitude is fine for an employee. It doesn't work for the CEO.