No she isn't.
TERF standard for:
Trans
Exclusive
Radical
Feminist
J.K. Rowling is not a radical feminist. She had never cited or referred to the works of Andrea Dworkin or Catherine MacKinnon, or promoted batsh!t things contemporary radical feminists do, like a curfew on men or an additional tax that only men pay.
A TERF is a specific breed of second wave feminists, and a rather nasty one at that. It doesn't mean "someone who has said something trans activists don't like."
Fine, she’s a TELF, Trans Exclusionary Liberal Feminist. Happy now?