J.K. Rowling is an "initial-heavy" author. What is mean by "initial-heavy" ?

That is a term used to describe someone who presents their name with multiple initials, rather than just a first name or a first name and a middle initial. J.D. Salinger, J. R. R. Tolkien, G. K. Chesterton, C. S. Lewis, A. A. Milne - the list is long!

https://en.wikipedia.org/wiki/List_of_literary_initials

I don't really think that using the initials instead of her given name (Joanne) makes her "initial-heavy". Instead it's the rather impressive letters used after her name: J. K. Rowling CH OBE HonFRSE FRCPE FRSL.

In order, these stand for:
CH = Companion of Honour
OBE = Officer of the Order of the British Empire
HonFRSE = Honorary Fellow of the Royal Society of Edinburgh
FRCPE = Fellow of the Royal College of Physicians of Edinburgh
FRSL = Fellow of the Roycal Society of Literature

The WikiPage for Ms Rowling contains clickable links that explains more what each honor is about.
https://en.wikipedia.org/wiki/J._K._Rowling
https://en.wikipedia.org/wiki/Companion_of_Honour
https://en.wikipedia.org/wiki/Officer_of_the_Order_of_the_British_Empire
https://en.wikipedia.org/wiki/Honorary_Fellow_of_the_Royal_Society_of_Edinburgh
https://en.wikipedia.org/wiki/Fellow_of_the_Royal_College_of_Physicians_of_Edinburgh
https://en.wikipedia.org/wiki/Fellow_of_the_Royal_Society_of_Literature