The problem is where you draw the line between "appropriation", "exchange" and on if wearing a costume to look like a X is either.

Firstly, as an example, because I love weird internet communities, consider this:

www.reddit.com/r/tulpas.

What you may note about this community is that it is really fucking odd. People trying to make imaginary friends, having big discussions on it, and so on.

What you may not notice is that it is derived from what you could call a case of cultural appropriation. Specifically that this community, the whole concept of creating a tulpa, is derived from some old book where someone found a bunch of Tibetan monks and wrote about their practices, one of which innovated the creation of a "spiritual guide" or a tulpa (other names were used). From there it got popular on 4-chan, then on the my little pony 4-chan board.

/r/tulpas, and similar internet communities have, on the surface, taken this deep and meaningful practice and "americanized" it, stripped it of most it's meaning, and turned it into a "companion creator for lonely people". Very few who join into that community are aware at all of what tulpa really mean, and what tulpas were originally. It's a somewhat sacred cultural practice twisted and squished into a secular view.

But if you look a bit deeper, you see an interesting trend, specifically that this practice has come to mean something for all the people in that community. Many are the sort of people who have some sort of mental issue in the first place, seeking a community that actually seeks to become like they are, when they may have thought they were damaged all along. The practice of making a friend, even if it is a hollow gesture in my opinion, seems to offer a huge comfort and draw to a large number of people. In many ways, it's become a religion all on it's own, a group of people with a central practice and a core ideal, coming together over a common theme.

But it's also a great example of cultural appropriation. Tulpas, to these people, is something stolen from a disenfranchised and misunderstood culture.

So, good or bad? Appropriation or exchange?

You don't get cultural exchange without cultural appropriation. Culture is not some monolithic, unchanging thing. It's the actions of people based on their upbringing, their environment, and so on. You can't have exchange without the twisting and destruction of the original values in some form. And if a person grows up knowing the term which was "exchanged" then that's the original meaning of the term, and will continue to be forever. That cannot be prevented.

But exchange of ideas, of practices, is a positive thing. It shouldn't go away.

___

Consider Halloween. It's a time to dress up as miscellaneous crap in a stupid funny manner. The whole concept is a tradition stolen from past cultures that died out due to the actions of churches past. Witches are part of an actual, ancient, religion. Vampires a production that belongs to Romania. Zombies an ancient concept as well. Strip away the appropriation, and you are left with what? Candy corn and consumerism? Pranks played by kids on houses?

When a person dresses up as a "Mexican" or as a "Ninja" or as a "Witch" they aren't "appropriating" anything at all, they aren't making those things meaningless, or attempting to make them any less serious or sacred than they are. They only are making the statement "this is what I think of when I think of Mexican culture". They aren't saying "these things are meaningless", they are saying "these things are Mexican". Yeah, perhaps it dilutes the meaning of such things, perhaps it encourages people to view places with stupid steriotypes, but if it weren't for Halloween, people likely wouldn't be aware of the practices at all. Without Mexican costumes and restaurants, it is likely the vast majority of the US would think of mexico as "beaches and cartels" rather than thinking of "stereotypical Mexican music and maracas". If it weren't for the "Indian with a headdress" we wouldn't be aware of that aspect of Indian culture at all. Without those costumes, it would be some boring, pointless, bit of history that almost nobody cares about.

How do we know that such actions "marginalize" a culture rather than increasing awareness of it? How do we tell if people would or would not be exposed to zombies, wiccans, indian headdresses, and so on without such topics?

We don't, not so far as I am aware. We just have people angry that "their" culture isn't uniquely followed or used by the people they think it's supposed to be followed by. Perhaps their feelings should be respected, perhaps we need to be setting up the social standards/constructions in society that prevents them from feeling their culture, this identity separate from all others around them, is important. However, they go against the long-standing trend of what happens to all cultures. They either die, or are transformed into something new as the world changes around them. You either bend or you snap under the pressure, I'd rather we bend.

Of course, I don't really have much of a culture to "disrespect" myself, and I don't dress up for halloween at all. Maybe I just lack perspective into the motivations and thoughts of both groups?