It is generally not recommended to add cement or any other form of topping to a dry stone wall.
Dry stone walls are built by carefully selecting and arranging stones without the use of mortar or cement, relying on the weight and interlocking of the stones to provide stability. Adding cement or other toppings can affect the natural drainage of the wall, and may prevent moisture from escaping, causing damage to the wall over time.
Furthermore, dry stone walls are traditionally built to be flexible and able to adapt to movement in the ground, which can be a common occurrence in areas with shifting soil or frequent freeze-thaw cycles. By adding cement or other toppings, the wall loses its ability to flex, and may become prone to cracking or collapsing under pressure.
Instead of using cement toppings, any repair or maintenance work on a dry stone wall should be done using the same traditional techniques and materials that were used in its original construction. This may involve carefully selecting and placing new stones to replace damaged or missing ones, or using a small amount of traditional lime mortar to help stabilize the wall. It is always best to consult with an experienced dry stone waller before undertaking any repair work on a dry stone wall.
HOWEVER... There are some situations which warrant cementing your toppings on.
In the picture below I am working on a dry stone wall. The far side of the wall dropped into a neighbours garden, about 15 feet lower. The wall owner was worried that if a topping came loose it could cause injury to anyone walking below. Therefore we decided to use mortar on the topping.
I have also cemented toppings on in the past on walls which have expensive, half moon shaped toppings. These are expensive and have been stolen in the past.