Generally speaking for practicing a religion, it's only appropriation if it's a closed religion (like Native American beliefs) where you have to actually be invited into the group to participate.
Buddhism is strictly not a closed religion.
Of course, as with all things "generally speaking", there are exceptions. Just copying cultural garments (especially sacred, religious ones) because you think they look good (or you think it makes you look hot) is also appropriation even if it isn't a closed religion.