Recent biographies have taken a more positive view of Grant's difficult presidency. The corruption that did exist didn't touch him, and he did what he could to dampen it. And his failure to follow through with Reconstruction was part of a balancing act to try to get the South re-united with the Union. It was not a commendable outcome, but it was an impossible assignment that he tackled as well as anyone could have at the time (arguably).
Only God knows, but there is much that he did well as President, given the circumstances.
As an ex-president, he completed his biography while suffering from throat cancer to provide for his family before the presidency offered a pension, and Samuel Clemons/Mark Twain did a good job of selling the book widely and with financial success.
He came a long way from being a penniless ex-Army-officer who ended up selling firewood off a cart to support his family, before being called back into the Army and rising in the ranks with astonishing successes in the West while the Union Army in the East was bogged down and beaten by Lee.