I don't watch GOT but Ed Sheran in the UK as popular as he is kind of has smaller roots that makes him a real thing. I actually have a friend who brags about knowing him form his early stuff (which tbf he did as I don't follow pop music and knew about him years and years ago) but it was never particularly good or interesting.

Beiber is much more hated, Sheeran is just kind of a big musician now that people may not like but they don't dislike him as they would others. I would say that other UK people have mentioned it was a weird thing for him to appear in the show.

It's just a PR thing that when the show is that popular already seems a bit odd. It will have definitely drawn in more viewers for the fist ep though.