Snopes: No, This Ironic Greta Thunberg Sticker on a Flooded Car Isn’t Real
... the image isn’t real.
The real photograph was included in a report published by the German news website Bild, where the submerged car can be seen without the sticker on its window, as noted by another Twitter user who included the real photo in a reply:
https://www.snopes.com/fact-check/greta-sticker-on-flooded-car/
The floods themselves and massive damage caused around the world by them, as well as wildfires and other ongoing hazards, are very, very real. And Greta Thunberg is absolutely correct to be mad as hell at older generations, my own included, for not doing enough about them.
(Where fault lies and more pointed blame are further questions.)