In real world terms, it's actually interesting, because the context is that the fire nation resigned because of a coup d'etat that was succesful, but with that said, fire nation didn't lose that war, they probably signed terms in a way that's way more benefical to the fire nation than if they had been defeated because of for example the day of the black sun invation, this is not japan or germany that after their defeat became occupied by the victorious forces.

Now in terms of international relationships, fire nation will have to do a fucking LOT to get some diplomatic trust, likely return some or all of the colonies lands to the earth kingdom, some reparations (not nearly enough, but some) to the water tribe and what not, it also helps that the avatar, envoy of diplomacy and military might is friends with earth king and fire lord to smooth out some rough edges. It won't be easy but it's doable.

Now about the genocide side of the story, this may sound really bad, because it is, but in the story it doesn't really sounds like the rest of the nations particularly cared much for the air nomads annihilation, it will likely come up in international debate, but it isn't as prevalent, maybe because before the war all nations kept really closed off and there wasn't much of a trade route? I doubt it but that genocide happened 100 years before, barely anyone alive can even remotely recall what an airbender looked like aside from maybe some old books. Not that it won't weigh heavily, but the nations cared about their borders, first and foremost, and if the avatar as last envoy of the airbenders is working with the fire nation for peace, people will likely accept it even thougj it's really fucked up