Well, Miley must get lots of people sending her messages constantly, trashing her and being plain mean to her, and she chose to reply to Sinead's because obviously it got to her.
Miley mentioned Sinead as her role model if I'm not mistaken in one of her latest magazine interviews, I don't blame Sinead for giving her some advice (which she could've just ignored), specially if she was mentioned by Miley first.
And her advice is deeper than you think, and a lot wiser than your description. I recommend you to read it again. She's not calling her stupid, but just young, as we have all been once, being young and without experience leads to people that might take advantage of us.
Miley's response on the other hand, was pointless and immature. Making fun of someone's mental illness? Really Miley?
Oh, and talking about interviews, Amanda Bynes was never going to be the next Lindsay Lohan, according to her
1:05 to be exact.
Nobody knows what will happen with Miley in the future, and I surely don't wish that she becomes the next Lindsay/Bynes or whatever, but her attitude right now is terrible and as a role model she should feel ashamed of herself.