Trump has no idea what either he or Obama would or should have done about the Crimea/Ukraine. That's not important to him. All he knows is that Obama didn't 'do anything' (he did, he imposed sanctions) so it's something to he can 'throw out there' as a criticism about Obama's Administration. He sees Obama's high ratings as president, he sees his low ones. So instead of trying to figure out how to raise his own, he seeks to discredit Obama's popularity as president.
He really, really thought that he would be 'the most popular, ever' because he's been a bully surrounded by 'yes people' whose livelihood depended upon him for his entire life. Bannon and his ilk continued it to get him into office to push their own white, rich agenda. Trump simply can't handle the fact that the majority of We the People, the MSM, and a growing number of members of Congress can and are telling him to Fuck Off, BIGLY.
What's wrong with us? Why aren't we adoring the new Messiah? Can't we see how bigly great his plans are? How making the rich richer and in effect putting the poor into 'uninsured death camps' (because many will die unnecessarily due to lack of insurance) will make things better for the middle class?
He's a sick, sick egotistical puppy.