Murky background, but discrimination over political views can be illegal in CA.

Being merely dressed as trump, imho, does not rise to political activity -- you're dressed as the current president, and there's no advocacy involved.

In fact, if you get shit about it from other employees, their opposition to you merely dressed up for Halloween as him would likely constitute political activity that affects your ability to do your job. Your employer would likely have to take action against them, and not you (as long as you are not advocating a position).

IANAL but At least that's what I've gleaned from the below:

https://californiaemploymentlaw.foxrothschild.com/2018/10/articles/discrimination/does-california-prohibit-political-discrimination-at-work/#:~:text=So%20employers%20in%20California%20are,on%20political%20activities%20or%20affiliations.&text=Political%20belief%20is%20not%20a,state%20and%20federal%20discrimination%20laws.