is it directly harming anybody? no, and whoever says that it is should get the fuck off the internet or her social media pages at the very least.
Are you suggesting that if you grew up and saw your mum had not only posted all of this about you, but encouraged many others to do so as well, it wouldn't have a profound impact on you? Peaches is pubicly proclaiming to hate her daughter, to the extent she wants her raped and murdered.
You could argue that Cora may understand it was due to mental illness and is no reflection on her (and I really hope there will be a plan in place and a network of support for her discovering all of this information), but this is something a lot of people struggle to truly believe (you can logically know something is true while feeling like it isn't, which can still be extremely powerful, eg: people with OCD often logically know carrying out rituals won't prevent something from happening, but the feeling that something bad will happen if they don't can cause them to engage in compulsions so much their life is completely consumed by them and they can be driven to suicide).
This is one of the ways mental health problems can cycle through generations: parent has mental health problems due to their own circumstances, the behaviour of the parent due to these problems causes the child to be more at risk of mental health problems (along with the possibility for genetic predisposition, which, according to the diathesis-stress model, the stress of the behaviour of the parent can be a trigger of) and if they do, the cycle continues.
Also, the idea that children following Peaches and becoming so desensitized to the idea of a little child being raped and murdered that they are calling for it themselves, even theoretically, not potentially harming them is ridiculous.
A lot of people who are mentally ill (and I agree Peaches seems clearly mentally ill) do things that aren't good and harm others as well as themselves (and even if they are "only" harming themselves, that's still a massive issue). Is it important we understand the behaviour so we can understand the root cause of it and hopefully treat it? Absolutely. Should we be excusing it or dismissing the effect of it to any degree? Absolutely not.