The perpetrator of physical child abuse is described in AMBOSS as being a primary caregiver but it is not specifically stated that it is most commonly the biological mother.
Firt Aid for the Psychiatry Clerkship also specifies that the perpetrator is most commonly a first-degree male relative such as a parent (biological or step father), guardian, or mother's boyfriend.
I have included screenshots of my 2 sources as well for reference.
Photo credits:
Extra: AMBOSS>Child Maltreatment>Physical Abuse:https://next.amboss.com/us/article/mP0V2T?q=child%20abuse#ANcRd10
First Aid: First Aid for the Psychiatry Clerkship (2022, 6th Edition)>Ch 10 Psychiatric Disorders in Children>Child Abuse https://neurology.mhmedical.com/content.aspx?bookid=3483§ionid=286949972#1207968646
Source: AMBOSS - https://next.amboss.com/us/article/mP0V2T?q=child%20abuse#ANcRd10