Typically a cat will lick and overgroom this area because they are itchy. Itchiness of the ventral abdomen can be associated with parasites (less likely in her case), stress (again, this tends to be in anxious cats so unlikely) , allergies or an infection.
Allergies typically present between the age of 1-6 and can be to a range of things including food, grass, dust mites etc.
Cats can develop fungal infections (such as ringworm) or bacterial infections. While we can never tell from a photo alone, I would be a little suspicious that this may be a fungal infection given the crusting and darker ring on the outside.
As she has licked fur off and the skin is pink and thickened, there is certainly an issue.
Putting a cone on protects the skin but doesn't address the underlying issue.
A vet visit is best so we can examine Mai. The vet may take a sample to check for an infection. Treatment will depend on what is going on. For example, she may need some anti itch medicine to break the itch scratch cycle as well as some anti-fungals.
At home, I would bathe the area using Chlorhexidine. This can be bought from any chemist. Dilute it 1:10 with warm water and apply with cotton wool. Leave on for 5 minutes and then gently rinse with wet cotton wool. Do this daily for 5 days. This should help reduce bacteria and fungal levels and can reduce itch a little.