If in the mod we play as an android, why is the player skin still the same? Couldn't it be some sort of a star-wars like droid or something else? Maybe even a spidertron?

It's a full body suit of armor and face mask, whose to say it's not an android? Some players already thought of it as a robot in the vanilla game, so it doesn't require a lot to reframe it as that. Nullius does not use a lot of custom graphics assets. It mostly remixes existing art resources from the vanilla game and other mods.

The character entity has 128 different graphics files, many of them containing hundreds of frames of animation each. If Nullius were to start adding custom art assets, that would be just about the hardest thing in the game to start with. I don't have the capacity to make that art. It might make sense as a standalone mod to reskin the character if someone else does have the capacity and desire to do that. I'd like the option of a high quality new character graphic if it existed.

If it's about player skins:
How about the "I, Robot" (turns char into compliatron), "Mechanicus" (WH40k)?
I think there was also one that had a flying drone design, but i couldn't find that one again. (Might be outdated, or was part of a scenario,...)