There are no Asian heroes in this story. I thought Kim's actions throughout were the most heroic of anyone in the story.
In the end, no one wins. The story is a tragedy. Like many others about all kinds of people.
Also, Chris' regret in the hotel room ("Christ, I'm an American / how could I fail to do good") is too little, too late. Actually, we learn earlier that he has been haunted with regret for years. The show first hints at his troubles before Ellen's entrance in I Still Believe.
Tam probably goes back to 1975 Atlanta as a mixed-race, out-of-wedlock child who just lost his mother. The Engineer doesn't get to America, but even if he did, the American dream is probably not what he ever imagined. John works for a NGO helping Amerasian kids and just watched the mother of one die. Ellen is the third wheel (in more ways than one). That's a lot of presumption outside the framework of the story. It really sounds more like the problems you have with Miss Saigon is that it is a story that simply should never have been told.
Ellen and Chris come in as the white saviors of Tam, but they don't really have that revelation. Except it doesn't actually play out that way. It's actually underscored with a lot of PTSD, stress, anxiety, tension and reluctance.
I don't think the show intentionally attempts to glorify america or american culture I don't think so, either. The biggest production number is a laundry list of American stereotypes. Chris is portrayed as loving, but not really a hero. John is American and fights for a cause. He probably comes closest to be the "American Hero" (and depending on the production, he's either white or black), so I guess there's a case to be made for being offended that it was an American that came the closest in rescuing Kim rather than a post-war Vietnamese citizen getting the job done. But as Wildcard mentioned, the creators were adapting Madama Butterfly to a more modern era, chose the Vietnam War, and went from there. It sounds like you simply did not like their choice in source material or the setting of its adaptation. Which is fine, but doesn't really support a case for the issues within the story, but rather, the entirety of the story and its existence.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian