Honestly, unless you're ethnic Japanese and born in Japan the only Japanese who might give you any trouble (and I doubt it'd be more than much) are some of the far-Right nationalists who aren't fond of the presence of foreigners (especially Americans and the American military). But from everything I've heard and read, unless you're making an *** of yourself you just really shouldn't have anything to worry about.
I don't know much about SK, but Seoul is an international hotspot, so unless you're causing trouble I just don't see there being an issue (not unless they do go to war against NK and they restrict incoming travelers).
And most people, throughout the world, are very aware the American government isn't an overall good reflection of American people. A MAGA hat might draw unwanted attention in various parts of the world, but I doubt any nation that wasn't already hostile towards America/Americans is going to suddenly be reluctant to allow Americans in.
Yup. And no doubt Kimmy is using the experience for his propaganda machine. It's unfortunate, and equally not surprising, that Trump doesn't realize there is a reason past administrations have pretty much ignored NK. The message is anti-American as it is, and here along comes Trump to confirm the threats of war, bombings, and a refusal to work with NK.