If the artists using elements outside their own culture do so with respect, fully acknowledging where their influences come from and giving credit where it’s due, that’s syncretism.
If it’s the example you gave, “hipsters in native American headdress at a festival,” who wear them without any concept of the history or context of the items, only doing so because “they look cool,” it’s cultural appropriation.
I’ll admit the issue is far more complex than that… but I think it’s a good baseline to start from.
