If it were Turmp spouting the same type of stuff about her that Sinead did, would you be defending him or would you be saying "yeah he's mentally ill"? Its not like Cyrus said that about her out of nowhere (Sinead literally called her a prostitute and rambled on about her in an open letter). Also, weird that you're trying to condemn people for outrage when Sinead ripped the picture, but want to cast Cyrus "forever into outer darkness" after Sinead is the one that instigated the whole thing.
You do realize both of them were actually darlings of typical conservatives early in their careers, right? That was what built their fame in the first place, so taking very open and vociferous stances opposite of that actually potentially could harm their careers. Plus what galvanized Taylor Swift was that her not saying anything while other people took shots at her (politically speaking, she took barbs from both sides too, as I recall quite a few female musicians criticizing her for not joining them on speaking out about certain issues) enabled white supremacists to claim that she was one of them and was seeding their ideology in mainstream culture (I'll have to wait for someonesmind to inform me if that was all just a 4Chan hoax or not though).
I don't think most people think about Sinead much at all. Hell her ripping the picture overshadowed her entire music career, which is a shame. As is her struggles with mental health. That doesn't give her a free pass to say things about other people without repercussion. Not that I'm defending Miley Cyrus (or even give a shit about what amounts to some dumbass twitter fight, I had to even look up what actually happened as I just remember having to roll my eyes about the whole thing when it made the news back when it happened).
Oh yes, because you Brits have never done that.