My mom is diabetic under control by diet and pills. just tested her blood sugar over 300. should she go to the hospital?

Diabetic control: If your mother has a glucose of > 300 mg/dl, she is not controlled. She needs to go see an endocrinologist and probably will need to start bolus-basal Insulin therapy. She should go to the hospital is she is urinating very often, losing weight and feeling ill. A good indication of her glucose control is her hemoglobin 1 ac. Ask her doctor, what it is. If it is > 7.0, she is not well controlled.

If pills only, yes: Unless she has a regular Insulin sliding scale to use to bring down climbing sugars, then yes. Sounds as though she should have tighter diabetic control and may need Insulin in addition to meds.