Coming from a family of diabetics, not necessarily.
A hypo (meaning low sugar levels) can cause things like fatigue, confusion, weakness, blurred vision and severe hunger.
Depending on the person (and whether they have type 1 or 2 diabetes) a hypo is generally when the blood sugar is below 4mmol/L.
If the blood sugar levels reach 0mmol/L then yes, the person will go into a coma.
The signs of a hyper (meaning high sugar levels) can be similar.
When the blood sugar exceeds 15mmol/L+ (my brother went well over 30 and it wasn't pretty!!) the person may experience severe thirst, urination, hunger, blurry vision and in severe cases will also go into a coma.