I didn't say her voice shouldn't be amplified necessarily.
I do, however, agree that her opinion (and the opinion of lots of famous people) is given more weight, public attention, and airtime than, say, a safeguarding expert who's worked in the field for years.
This is case for almost anything - if Judi Dench says "Aldi yoghurts are shit", that's going to be a Metro article. If I, a (hypothetical) dairy produce quality expert, say actually I love Aldi yoghurts, nobody cares, because I don't have the fame attached. Does that mean her opinion on it is more important? No, it's just got a bigger audience. But if 100k people retweet her saying they're shit, and a few papers do a piece on it, and Dawn French and Steve Coogan come out and agree they're shit, then in the general discourse, Aldi yoghurts have now been deemed shit - because the voice given the platform is heard.
It is undeniably true that being famous and weighing in on an issue has the ability to a) drown out voices without the same platform and b) sway or change the conversation in the public eye. It distills nuanced conversation down to hot takes and tweets and throw-away comments and makes it about A Celebrity Opinion rather than the debate as a whole.
Any celebrity has a right to an opinion on any topic. But their opinions often dominate a conversation (or how it plays out in the media) and that's very rarely a good thing in my opinion.