This is the 21st century! I know Korea still live in the past, but Idols has to know about racism, especially because it's happening with Asians as well! So, yeah, being criticized for doing it is okay. Everyone can make a mistake, but don't do it twice.
Mamamoo did blackface too back in the days, they apologized, end of story. People criticized Wendy, too.
Wendy imitated how black people speak. That's not racism, just rude or ignorant. A lot of people do that. White comedians make a living doing shit like that.
There's a huge difference between intentionally using racist slurs and doing it for fun, like Mamamoo or Wendy did it. In my country, there was a stupid music competition show where celebs had to cover famous songs, and they had to look like the artists, and they did blackface many times, but nobody cared.
There are cases when people go overboard, and think everything is racism. Like when people criticized Ahyeon using that slur on her live show. That's not racism, she just covered a song, that's all. Or when people were really toxic with Hwasa because she wore clothes that usually African people wear. People said she's racist because she wore a red scarf, or whatever its name is. She did that many times, because she loves to wear clothes like Rihanna or Beyonce, her faves. That's also not racism.
For the record, I'm a POC, too, so I know how real racism looks or sounds like.