I myself have been a diabetic for more than two years. Since diagnosis with A1C test number of 7.5, I've exercised and watched my diet daily. My A1C test results have reduced and never been over 5.2. Now I am very happy and healthy than ever. All I've been doing to control my blood sugar is read and and follow this book, "Dr. Bernstein's Diabetic Solution."
My husband has had diabetes for the past 20 years. He also feels fatigued. He gets cranky & flies off the handle easily when his sugar is not under good control(which i've been told is chartaristic in diabetics). Sounds like she needs to seek additional help from her doctor. Her blood sugar needs to be better controlled.
"Is it her blood sugar being so high that could cause this?"
Yes, uncontrolled and unmanaged diabetes will cause fatigue. When was the last time your mother had a full health exam? Along with the possibility of having her diabetes medication adjusted, the doctor may discover other underlying causes for her fatigue.