To me, something like this can be considered akin to a southerner wearing a jacket with the Confederate Flag on it in the name of "Southern Pride". In the sense that a shirt to some carries a positive connotation but ignores the implications if you think about it for more than 5 seconds, but the relations between Japan and Korea are a lot more heavy and politically-layered than that.
First of all, I highly doubt that Jimin is some sort of extreme Nationalist just because he wore a poorly designed shirt that carries unfortunate implications.
The tensions between Japanese and Koreans still exists to this day. My Nana who is a native Japanese person who immigrated to the US in her 20's (she's in her mid-80's) absolutely HATES Korean and Chinese people. The movie Big Hero 6 had to be censored when it was screened in Korea because of the rising sun flag that was used in the background. The xenophobia that foreigners face in Japan is still very real and very visceral.
As someone said below, Korea had been under the rule of Imperialist Japan at that point for decades and nearly had their entire history and culture wiped out by the Japanese. It's easy for us to sit here and pass judgement and write off what happened in Japan as a tragedy and that in hindsight the bombs shouldn't have been dropped, and so on. But it's easy to say that when you're so far removed from the time that WWII occurred during Total War, and especially in the day of precision drone strikes. Most of the Western world is aware that the entirety of Germany is not responsible for the Holocaust because the Nazis and Hitler were the ones who carried it out. Japan did not have a group like that to pin the blame on so the entire country is deemed as being responsible and complicit in the atrocities that Japan committed.
There seems to be this weird attempt of rewriting the narrative of the dropping of the atomic bombs to try and frame Japan as an innocent country caught in the crossfire by the evil United States but it just isn't true at all. Hiroshima and Nagasaki were not complete civilian cities, these two places in particular were involved in the war and the creation of weapons. Japan was one of the Axis Powers, they sympathized with the idea of a superior race and did not see Chinese, Filipinos, Koreans, etc. as being equal to them and brualitzed and tortured their civilian populations and POWs in horrific and inhumane ways, they tried to conquer as many islands and countries as they could in Southeast Asia. Yes, not all Japanese civilians were responsible for what the army had done during WWII. Just like not all Germans were responsible for The Holocaust. Holocaust Denial and Nazi imagery is banned in Germany and you can get arrested if you try to deny the Holocaust actually happened. To this day, Japan still has barely apologized or even taken responsibility for their war crimes. The Rape of Nanking, Unit 731, "Comfort Women", etc.
Maybe "deserve" is the wrong word choice here but the bombs were a necessity to bring an end to WWII as fast as possible with fewer casualties than there would have been had the war been dragged on longer, and they were not war crimes. The Tokyo firebombings killed more civilians than either of the two casualties combined.
The design of the shirt is pretty YIKES but when you take everything into account it's not hard to see why a native Korean (compared to someone from the West) would see the droppings of the bombs as a good thing as it freed their country from the brutality that was enforced upon their people by the Japanese who saw their kind as subhuman, and swiftly brought to an end the war that already killed dozens, if not hundreds-of-millions of people.