INZ require people to do medicals when they're applying for the kind of visa when they will be in NZ for some long time, and may in due course become eligible for health care from the NZ Health Service. People on visitors' visas don't get this except in an emergency, and need to have their own medical insurance in case they need any minor treatment while in NZ.
Your mother can apply for a visitor's visa if she meets the conditions.
https://www.immigration.govt.nz/new-...-visa#criteria Her state of health doesn't come into it unless she comes from a country where TB is endemic, or if she's hoping to stay in NZ for over 12 months. She would have to carry her own medical supplies, and have insurance.
If she applied for a parent/grandparent visa, she would have to have a medical.
https://www.immigration.govt.nz/new-...-visa#criteria Diabetes which is under control by well-known medications is no barrier to being passed as having an Acceptable Standard of Health (ASH).