It has it's own lore and aesthetic and the setting is Pop Science Fantasy. That doesn't mean you can't take the rules and play any kind of game you want, but it is very much intended that you play heroic space adventurers fighting evil space monsters.
Why would you try and make Starfinder into Alien?! It's not at all designed for that. If you want Alien play Mothership. For Star Trek, Star Trek Adventures is already in existence. Starfinder is a specific genre of Science Fantasy, no need to try and make it something different when other games...