To repeat something that has been said a lot over the past few years, JK Rowling is not just some boomer on Twitter with stupid opinions. She has the equivalent of the entire population from Rwanda following her and is the creator of one of the most culturally significant works of fiction of the 21st century. She is hailed as a literary and cultural icon. We have seen that she has a direct impact on policy in the UK. And that's before getting into the fact that she is a billionaire who can and does donate insane amounts of cash to transphobic causes.
So when she tweets blatant misinformation and nastiness about asexuality, a sexuality that is already misunderstood, underrepresented and mischaracterised, it is for a good chunk of her audience one of the first times they've heard of asexuality, maybe the first time for a number of them. As an ace person, it is not comfortable knowing a significant number of people went from not knowing my community exists to viewing us as self-absorbed wannabe victims with no real oppression or problems. And while they are not passive victims of Rowling's manipulation, her incredible influence means she is bringing asexuality into the mainstream under this light.
So no, there is no "just separate the art from the artist", nor do I feel I can sit back and let her yell into the void when the void has 14.5 million people in it, actually many more who aren't on Twitter. I don't know what the so-called "correct" action to take would be, but it isn't brushing her off as just an old biddy who doesn't know what every pride flag means. She shapes how communities are viewed by wider society and is wielding this power to harm us.