I think Obama's ratings will follow a trajectory similar to that of Ulysses Grant's. Grant was thought of as a terrific president when he left office and for some years after, but as time went on his popularity faded and his approvals sank to the lower end of the spectrum. I think people who did not live through the Obama administration will eventually see that his economy wasn't great, that his reelection was slim, that the two midterms over which he presided were terrible for Democrats, and that he ultimately failed to pass the torch to his hand-picked successor.
The last 3 are totally irrelevant to Obama's performance as President. And the economy was fairly good at the end of his term. He was handed the worst economy since the Great Depression, yet he got us out of the recession way faster than FDR. Obama will look great in history as the President who helped get us out of such a horrible great depression despite facing such big opposition.
Obama was overall a good president, even if I didn't always like what he did.
As for overrated presidents: FDR, Kennedy, and Wilson. Moreso Kennedy and Wilson than FDR.