I think it's good that she's highlighting the absurdity of people using asexuality as an identity.
The TRAs are very keen to make being asexual part of the LGBTQ umbrella because they know that all the poor kids like Jazz Jennings and Jackie Green who had the misfortunate to be given puberty blockers will never be able to experience sexual pleasure.
If they can just claim that it's no big deal because loads of people are 'ace' then it detracts from the magnitude of horror of everything that was stolen from so many kids. Thousands of young people will never know the joy of feeling sexual desire or sexual fulfillment or the happiness that comes with intimacy or the closeness of the bonds it forges. All of that was taken from them by ideologues.
This is the real tragedy, not the fact that JK Rowling is questioning whether or not a day to celebrate being 'ace' is appropriate.