I can't imagine what makes you think you can speak for Americans in general, particularly as I think history shows that assimilation is both expected and approved of among immigrant populations in America, and that most animosity towards immigrants in this country is expressed in terms of failing to assimilate, failing to participate in society.
Not that Jamie seems to be assimilating or attempting to 'change his skin' at all. Your perspective seems truly warped. He lives in America, he uses American slang sometimes and talks about things at times with references to American politics and culture. I don't grasp your need to turn this into him somehow trying to be American or not proud of his mother's culture, the one he was raised in.
As far as his comments on the difference between how LA and Britain treats its actors and budgets its productions, it's simple fact that there is a difference in production values. Jamie has a tendency to be snarky, but he is far from always going on about it, though he and other actors who have worked in both areas do talk about it. When you consider that one of the big differences between the industries is that in America those big budgets lead to much longer TV seasons with more work and stability for those actors that snag roles in them, as opposed to the painfully short British TV cycles, I think any British actor has to appreciate the money aspect of American media, at least, whether or not you've happened to catch them admitting it in an interview.
You seem to be ignorant of his recent career and living situation, among other things. He's been living in LA for years, he splits his time between LA and England in terms of acting. His success as an actor, like the success of most actors, in any country, will be based on his perceived attractiveness, his acting skill, and his luck in being offered or choosing parts. Jamie's not a fresh-faced unknown trying to make it in Hollywood. Casting a 37 year old man does not happen because of the acting reputation of his country of origin. It happens because that actor himself has a body of work that either does or doesn't impress the director and because he as an individual does or doesn't seem like a good fit for the part.