It may be helpful to describe what you (or your mother) are doing in terms of serving/eating nutritious snacks and meals. Then ask her to explain why nutrition is so important. We suspect she is making her recommendations based on current best practice from research findings.
Specifically, poor nutritional status in patients is associated with a trend toward wound infections. Doctors use a blood marker called serum albumin as a measure of nutritional status. Low levels of serum albumin is found in patients with infection.
Since wound infections can delay recovery and sometimes even result in further surgery, nurses do everything they can to help patients prevent this potentially serious complication. Your questions may help her see the need to explain this more clearly to all her patients. You may even want to ask what your mother’s serum albumin levels are and how that compares to the normal standard. This may help you understand the focus on adequate nutrition.