The mask is a caricature based on racial stereotypes of black people. Specifically the large teeth, ears, and nose. It is this depiction that is steeped in racism, as racist caricatures of black people have been used against them for a very long while.

Additionally, there are issues with privileged people of any race changing their skin tone as a costume, either with paint or a mask, because it reinforces that something that is an excuse for marginalization of an entire people (their skin tone) is a costume one can put on or remove with no consequences for the people that marginalize said group. South Korea, like the US, has a very real problem with racial discrimination towards black people.

The issue with her posting the photo was more one of ignorance or carelessness, in which she either didn't know or wasn't mindful to think about it before posting. That doesn't make her racist, but it also doesn't mean that the spreading or normalization of such imagery and stereotypes isn't harmful. This apology addresses that well. The thing that really riled people up was the non-apology telling people to "chill" (that, if we take her at her word, was unrelated) that followed.

Hopefully this helped.