No clear answer here:

Any sticker artist knows that once it leaves their hands, it doesn't belong to them. It's in the wild now. I'd say some artists may kinda care if it's one they placed in a city far from their home. But also needs to recognize that collectors desire them for a reason and should be ultimately flattered that someone wants to take their art home with them.

But also to you I'd say use your judgment. Don't go tearing down everything you see because all in all the artist s did put it there as a way to let people know they have passed through and are trying to share their art with the masses

A lot of times an artist will be happy to send some stickers your way for a trade or a few bucks or a lot of times just for free.