My experience has been that dislikes are station specific. I regularly dislike songs on channels in order to shape the genre, even if I actively listen to that song on other stations. Pandora's whole identity revolves around stations, so it would be weird if dislikes were global.

I've found the pandora community post below that seems to confirm it, though I'm not sure whether the community admin answering the question is actually a pandora employee or not.

https://community.pandora.com/t5/My-Collection/What-if-you-l...