His administration was very corrupt, which is a black mark that can’t really be re-evaluated away. Grant himself is widely accepted to have been innocent - the man died nearly penniless, and was never a prodigious spender - but he was trusting and loyal to his friends. These are actually really BAD traits for a politician, in which trust and loyalty are a big “USE ME” sign painted on your back.

However, Grant’s overwhelmingly negative reputation has a lot to do with the domination of Lost Causers in historical academia up until the 70s. He was positively radical on civil rights, crushed the First KKK, pursued a policy of negotiation and attempted coexistence rather than war with Native American tribes, set up reform within the civil service, was positively inclined towards women’s suffrage, created the country’s first national parks, supported public schooling, and elevated African-Americans and Jewish-Americans to high posts within the government despite the racism and religious prejudice rampant in the period.