Mars is wholly unsuitable as a place to colonize, and Starship would anyway be far from adequate to attempt it, by at least two orders of magnitude.

Starship might suffice to maintain a moon base, after the model of McMurdo Station, albeit radically more expensive. But that would be an end in itself, not any sort of staging point. There is noplace to go from there.

O'Neill cans make better sense. After aneutronic p-11B fusion is mastered in some future century, O'Neill cans out in the Kuiper Belt will be a place for humanity to expand. The inner solar system is a dump, Earth excepted.