Most non-Jews do not even understand what a kipah (yarmulke) and tzitzit really are, what they are for, and what they represent. They have no sense of context and meaning for these things. Which, ordinarily, is fine: non-Jews have no occasions which call for the wearing of these items, and thus no need to understand them. But when they put them on in order to portray either a legitimate Jewish character or merely a Jewish stereotype, then the need arises, out of respect for Jewish culture.
There is, indeed, a difference between a trained actor attempting to bring a Jewish character to life with authenticity and respect, and a random person deciding that they can dress up as a Jew for Halloween, as though "Jew" was no different than "spaceman" or "zombie" or "sexy nurse." That is not an instance of an artist trying to make a Jewish character live, that is an instance of someone stereotyping for a lark.
The fact that the person doing so may claim to have no offense intended does not make it better-- in some ways, it only makes it worse, as it reflects not only lack of understanding of the clothing or ritual items used as "costume props," and ignorance of the culture that produced them, but an ignorance of history, which is full of non-Jews exoticizing Jews, appropriating Jewish culture, and in various ways demeaning Jewish identity. All of which is in the background of someone feeling completely free to dress up as a Jew for Halloween.
Some others have brought up the argument that people sometimes dress up as members of different other cultures-- Greeks, Scots, American Indians, etc., and that is not seen as problematic. I cannot say whether it is or is not seen as problematic: I am not a member of those cultures, and I cannot speak for them. I would personally imagine that probably some members of those cultures, at least, do take offense at stereotypes of their cultures being used by non-members. But I do not know for certain, and it is not my business to try and rise to their defense unasked. What I do know about is that it is offensive to dress up as a Jew for Halloween. And not being able to see how it is disrespectful is, if anything, merely indicative of the degree of how devalued Jewish culture still is among non-Jews.