Being equally shit across the board isn't really an excuse. What stayed my hand ultimately was when I realised I might be looking at this from exactly the wrong angle. Rather than wonder how anyone could write such a terrible example of a Native American character after everything that had happened before, I wondered whether there was any way to reconcile Thunderbird's behaviour with that history.
The second generation blues
Our point of view not listened to
Different world and different rules
A question of allegiance
Where is it?
Where is home?
The second generation issue is a fairly common one in this country. How does one react to being born into a massively different culture to the one your parents grew up in? Some ignore their parents' culture, some embrace it to an extent incompatible with British culture, but for many, and probably most, the struggle comes in attempting to balance that which can't be balanced.
That's for those separated from their family history by distance, though. Proudstar is separated by time. Compounding the problem, his parents died when he was a child, leaving him to be raised by his grandfather, which probably didn't help matters as regards striking the right balance of cultures. It may not be any wonder that confused and pointless cultural references are par for the course. John Proudstar is a man searching for an identity, only he can't find it, because all he has is rage. Rage at the interlopers for stealing his country, and then rage at own people for not being strong enough to stop it from happening. He's so angry with Caucasians he joined the US Marines to prove himself, attempting to demonstrate his superiority by doing what the white man wants as efficiently as possible. A few years later, he does it all over again, signing up with Xavier essentially because the latter man called him chicken.
I guess it all comes down to three choices. Fight the system, ignore the system, or succeed in the system. Which is true for all of us, obviously, though some of us at least get to make the choice with respect to "our" system, whatever that means. I would be somewhat surprised had that been Wein's intent, but it makes sense, and it has the advantage of defining the character directly. Whilst, as I say, a degree of caution and understanding would have been desirable in creating James Proudstar, that really only gives you an idea of what you can't do. It's colouring in the borders, nothing more.
Anyway, Thunderbird goes completely for option 3. If the world is a game run by white men, he's going to make damn sure he beats them at it. Each new challenge is an opportunity to prove he's the strongest, or the fastest, or the most determined. Each success is crowed over, each failure leads to one of those bizarre outbursts of simultaneous sullen self-pity and needless aggressive fronting most commonly observed coming from hormone-crazed teenage boys. Ultimately, Proudstar's obsession with demonstrating his worth leads to him jumping aboard a departing Harrier jet, so as to get to its pilot, the villain Count Nefaria. Despite knowing full well that Banshee could have stopped the plane easily were he not in the line of fire, Thunderbird insists on destroying the plane himself, which not surprisingly gets him killed.
Maybe that was the intention all along, whether or not he realised it. Believing his last act a success, maybe Thunderbird didn't want to risk another. This battle he had won, but I don't think he ever believed he could win the war. The Apaches knew that early on in the hills of Arizona. The US Marines realised it quickly enough in the stinking jungles of Vietnam. We all know that, one way or another, we're going to fail eventually, and fail so badly we can't recover. There's just too many different directions we have to run, and too many measures by which we can fall short. Any situation in your life has a geometric distribution, you succeed until you fail. And a lot of the time, those successes aren't worth the price you pay for them, and the failures not worth the effort you put into fixing them. If you think Thunderbird was crazy for jumping through "Custer's" hoops, I bet it takes you all of thirty seconds to think of a time in your life when you tried to succeed at something for the worst possible reasons (I'll bet further that it involved someone you wanted to interfere with sexually).
Update: Mozz points out in comments that I was talking smack yesterday; Shadowcat is the first recruit of the 1980's. For some reason I'd forgotten Giant-Size ended a five-year hiatus.