(Note: “you” is a collective you directed to white people, not necessarily you, anon. Okay? Now.)
This isn’t henna:
However cool it looks, henna doesn’t exist for people to make ish like that.
Those apply-water-and-stick-on things also are not henna.
Both of then are ridiculously disrespectful to the art of henna and the history behind it.
This is henna:

Henna, as in the plant and its paste, can’t be appropriated, but the designs that go with it are. (Henna, the plant and paste, also have medicinal qualities, so obviously not). The design, and the use of henna with it, has ceremonial and cultural significance. As in, it’s applied on religious holidays and weddings and so on.
I don’t know if I can say white people wearing henna would be appropriative, per se, but a bit like with kameez shalwars, I feel finicky whenever I see white people wearing them. Henna is, like I said, applied before important events, and very often it’s applied in women-exclusive gatherings, so it’s an actual experience and I don’t think white people wearing henna could ever understand that (it’s a live-share-experience thing), or its significance, or the way we’re treated when we define our ethnicity or culture through our henna (or our kameez shalwars). (No, being called “exotic” isn’t a compliment). And of course, white people wearing henna often leads to disgusting things like this. And whenever things like that happen I go from “oh yeah it’s okay to wear henna” to “now I remember why I don’t want white people wearing henna”.
But whoever’s wearing henna as part of their culture can wear it any which way they like, it’s theirs to do whatever they choose with. Yes, even if they want to use henna as a temporary tattoo. My (older) cousin brother would get little designs on his shoulder before weddings and my mother and aunt would scoff at him for missing the point of henna but nope, it’s his culture, he can do whatever he wants. White people, not so much.
tldr; I wish white people would stay away from henna, because it isn’t just “art”, it’s cultural. Yes, I also think white people wearing henna look ridiculous. Sorry white people, you can’t actually pull off henna. Yes, I laugh with my sister when I see white people wearing henna. I’m obviously a mean bitch. Can’t help it. Yes, it’s okay to wear henna to PoC weddings and stuff. No, it’s not for for you to declare artistic feminine spirituality or whatever white girls do.