Can I use 14/2 cable for a light switch loop?
You can use cable that contains 14AWG conductors for adding an additional light on the circuit as long as the circuit breaker that protects that wiring circuit is not rated above 15A.

It there is a case where the existing wiring of that circuit happens to be 12AWG and the breaker is rated at 20A then any additional wiring runs you add to that circuit will have to be 12AWG as per code and safety guidelines. A 20A breaker cannot properly protect a wiring circuit that contains any 14AWG wires that may develop a short circuit or be exposed to an overload.