Is Madam Hooch in Harry Potter a squib?

No. Ariana was violently attacked by some muggle boys who had witnessed her practising magic and wanted to see it again. Ariana was only six and could not reproduce the magic thus the boys attacked leaving her emotionall scarred and unable to control her magic. The Dumbledore family then kept her hidden to protect both her and other people. Not many people knew that she existed and those who did believed she was a squib who was being hidden by a family who were ashamed of her.