She gets it, really. Tina didn't mean anything bad – the is one of her best friends, Mercedes knows her well enough to believe that. The girl was just scared; she came clean in the end.
But she gets the other half too. She does kind of remember that time in elementary school those jerks at the back thought it would be funny to dress up in black face for their skit at school camp; no, it's not the same, but she gets why Artie is hurting and won't talk to Tina.
Well, fuck. This isn't all kinds of complicated.