The Korean War was probably more unpopular even than the Vietnam War in the 20th century, it’s just not remembered as much. People didn’t do the same type of protest but no Vietnam war President saw his approval ratings plunge like Truman’s did, and Truman’s largely plunged because of the Korean War although economic issues also were at play.
The only reason Nixon’s ratings went so low were because of Watergate, he otherwise would not have seen his ratings get anywhere near as low as Truman’s.
But a few things have happened historically since that time. For one, people recognized the importance of Truman’s first term. Specifically his decision making at the end of WWII and how he much more assertively handled the peace and dealings with the Soviets than feeble Roosevelt did. Truman probably deserves a significant amount of credit for the Iron Curtain being where it was instead of perhaps hundreds of miles further to the West.
Truman also had a pretty decent legislative history throughout his term as President. Finally, while the consensus at the time was that Korea was a waste of American blood for no discernible reason, it’s recognized by modern historians South Korea and its current economic strength is 100% because of Truman. Certainly if Dewey had been in office there would not be a North/South Korea today, just one DPRK living under a Stalinist government. For many years after the Korean war South Korea was a dictatorship that had a pretty terrible economy, but from the 80s on it became much easier to put Truman’s saving of South Korea down as another feather in his cap instead of a failure.
I honestly don’t see any similar situation in which President Obama has acted. Instead his low approval rating is due to prolonged and steadily developing disillusionment with him as a President. If Nixon can rehabilitate his image (and he largely had by his death), then anyone can, but the biggest issue for Obama is his accomplishments are so limited there’s not much to build. To me it’s more likely Obama goes down as a Calvin Coolidge style President. Filter out the issues of race and the coalition that elected him and that’s a more apt comparison for Obama than Coolidge.
It’s arguable Obama is actually a worse President than Carter. Carter wasn’t actually all that bad on domestic policy, and his weakness at foreign policy at least showed that he had some backbone. Carter took much more forceful and rapid political actions against the Soviet Union’s invasion of Afghanistan than Obama has done against Crimea–Carter at least realized when “consensus building” no longer had any place in his foreign policy decisions.