IV therapy in hospice careis a controversial issue. Many research studies have been done on this topic. In many cases, IV therapy isn't an option for hospice patients because they have poor veneous access (no veins), or a health condition that makes giving fluids dangeous, like kidney disease. If this isn't the case with your mom, then maybe your question is more of a moral one.
Remember that the goal of hospice care is comfort. I'd ask myself, "Does my mother complain of thirst or discomfort from feeling thirsty or dehydrated?" If the answer is no, I wouldn't worry about giving extra fluids through an IV. I know that current research shows that giving fluids artifically can cause fluid overload, which is manifested by increased urination, swelling of the legs, and even excess fluids in the lungs. Also, the insertion of an IV can be painful. If there isn't any discomfort from thirst, the risks of giving IV fluids can outweigh the benefits. Now, if your mother is clearly uncomfortable and complaining of thirst, then I'd definitely talk to the hospice team to see what interventions can help reduce her discomfort. Hospice is usually very good at working with families, and I'm sure they'll help you address this concern.