I think it depends on what sort of content is posted. If it’s mostly promotional stuff some people may feel that it doesn’t fit the vibe of Mastodon. If it’s thought-provoking content (especially journalism) then it will be a win. Either way, having The BBC on Mastodon seems like a big deal, to me, and maybe it will induce other journalists to explore the fediverse.
I would say Mastodon already has. I’ve been spending a lot of time there over last few weeks and there’s more content than I can consume. Breaking news stories are covered well, including live blogging, although a lot of that content is cross-posed from Xitter. Plenty of people to follow, including authors, photographers, journalists and scientists. An increasing number of media outlets have a presence there, as well.
Xitter still has an order of magnitude more users, but Mastodon is mostly Nazi-free (which is nice).