Mindy Park's ethnicity isn't described in the novel. Weir has subsequently stated that he personally thought that she was Korean-American but that the movie casting was perfectly fair and equitable given her name and what limited physical descriptions were available to the casting agents and director.
He [Andy Weir] did admit that he’d always pictured Mindy Park as of Korean
lineage, but emphasized again that he had never actually explicitly
written her as Korean.
“Whatever ethnicity she has, she’s an American and her family has been
in America forever, which is why her first name is just Mindy, but her
last name is Park. But Park is also a British surname so the casting
people [could have] thought Mackenzie Davis looks like someone
descended from Brits,” Weir said, laughing. “And she did a great job!
I’m certainly not complaining about anything related to casting.”
ONE PERSON WHO DOESN’T THINK ‘THE MARTIAN’ WAS WHITEWASHED? THE AUTHOR