Err... Superman's an immigrant because the parents of both his creators were immigrants? Is that actually what you're saying?

His parents sent him to Earth to have a better life (any life at all really) - Right, that was my main dilemma. Not saying you're flat out wrong, but I think 'immigrant' exists in a cultural and social context. Let's go over some examples:

1. A woman gives birth on vacation
2. A family moves to another country has a child and then moves back when the child is 12
3. Having a child a day before immigrating.
4. Having a child a day after immigrating. I'd say 2-4 are immigrants, because they and society would probably view themselves as immigrants.

If he were human, the government would consider him an immigrant - Legal status is irrelevant.

Superman didn't have to translate things to his parents because he knew English better than they did, his upbringing didn't give him an outsider insight to the culture that surrounded him, he doesn't have an accent or not can code switch from English to Kryptonian, he isn't viewed as different by his peers. He was raised as a whatever-generation farm boy from Kansas.

He does have Kryptonian heritage, which would make him closer to second generation immigrant. Just like his creators.