Ask the doctor ...
Smaller appetites are a normal part of ageing, particularly if you are less active. However, a change in appetite should be investigated to rule out any underlying condition, so your mother should make an appointment with her GP, for an examination and a discussion.
Even with a reduced appetite, it is still important to eat nutritious food every day, rather than three large meals a day. Your mother might find it easier to eat three small meals, with nutritious snacks in between. Here are some general suggestions for her:

