I'm in two minds about it.
OTOH a few quibbles aside, almost everything Sinead says in that letter is true.
OTOH you can't offer advice to a young woman in public without it being seized upon as criticism, judgementalism etc. The tone of the letter wasn't critical or judgemental and I also don't think it was slut-shaming - she's using the term prostitute in the context of "prostituting our art", but inevitably, if you're going to go public with a conversation, you're going to provoke defensiveness and defiance in all but the most intelligent, secure and confident of people.
She might have been better sending this letter in private.
But then OTOH she wouldn't have got her message out to all the other young women out there who would also benefit from hearing that message - from having some of their unexpressed uneasiness spoken aloud.
It might have been better if she did a more general letter not actually addressed to Miley C because then it would be less personal.