I don't think there's any issue with voting for Jill Stein, RFK or Mickey Mouse as long as you're doing so with the full understanding that they will not win. As long as you've fully incorporated that into your mental calculus and it still seems to be the best course of action, do as ya like.
While I agree it's the right of every member of a democracy to vote for the candidate they think best, it's also important to not be blinded by naive ideology and participate in these incredibly flawed systems pragmatically and practically, as they do not work in the way they advertise and must be manipulated in order to achieve the most positive possible outcome.
Jill Stein winning a single vote in the electoral college is not a possible outcome.
That is ideological naivety. You are operating with a belief in the way the world should work but not with an understanding of the way it currently works.
Practically, this is almost a non-issue because as a single person you have so little impact on politics that your naivety is irrelevant. But it means that your actions may be counterproductive to your goals.