Going to Mars is easy compared to Fusion, just requires, say, $100 billion. It's been possible but too expensive for decades, the idea of Starship is to make it cheaper.
Then it's just a logistics problem.
Plus whoever founds the successful human civilisation on Mars gets into the history books, fusion is a massive team effort that won't have one specific person remembered.
Going to Mars is easy. We've even done it twice in the last 5 years.
Making a self-sustaining city on Mars is impossible. And even if not impossible, certainly costs many tens of trillions of dollars, which is as good as.
Impossible is pretty strong. Who knows, maybe there's material that can be used for construction buried 100 feet under the surface. Or we figure out how to build stuff with Mars dirt. Humans are the most resourceful and adaptable beings in the universe (that we know of) and life, uh, always finds a way.
Then it's just a logistics problem.
Plus whoever founds the successful human civilisation on Mars gets into the history books, fusion is a massive team effort that won't have one specific person remembered.