She is Chinese and not Thai and has a darker skin because China is a country that has huge territory that lies in different Climatic Zones. Her phenotype doesnt look Thai by the way. I saw once a Chinese Girl that looked similar to her.
As I know she is an ethnic Chinese from the Peoples Republic of China and has only a darker skin because she is from Southern China and has probably also Ancestors who belong sinized ethnic Minorties who living in Southern China.
At least 50% of the entire Thai population are ethnic Chinese, but identify as Thai which has been the norm for centuries. Chinese princesses were marrying Thai princes for centuries, and Chinese immigration to Thailand also for many centuries.