Yes, "mammy" or "grandmammy" is considered very much racist and malicious.Is calling someone a “grandmammy“ that malicious either? That was the other incident. I’m not all that familiar with the term, but it just sounds like a really country way to say grandma to me.
This woman was fired for doing something she was explicitly warned against & disciplined for in the past.
This wasn't racist, but it was dumb as hell and she deserved to get fired for violating direct orders from her superiors. It's a no brainer.