Short answer
A single lick is not an emergency. Sour cream is high in fat and lactose, which can upset a cat's stomach. Just keep an eye on her for any signs of vomiting or diarrhea over the next day. If she develops those symptoms, give your regular veterinarian a call.
Sour cream — Food safety facts
Feed with care · Cats · full breakdown on the linked food page
Fat and lactose — likely to cause diarrhoea in cats and sensitive dogs.
The full verdict, portion guidance and serving notes are collected on the Sour cream food facts page.