There is certainly more to say, but this question has come up here a couple of times before and picked up a gold-rated response by the mysterious u/deleted about a year ago. You might like to review that thread while you are waiting for fresh answers to your query:

Harry Truman held the lowest approval rating for a President that Gallup has ever measured, at 22%, in his final year in office, yet he is generally regarded as 'great or near-great' when historians are surveyed. What accounts for the discrepancy? What do we see that contemporaries missed?