I didn't know anything about this book before, but here goes anyway: seems fair enough to me to represent racist attitudes of the time, and use the language of the time to show how they were expressed. You can't change the past. What is much less acceptable is the caricatural working class language and the caricature of working class people you see in that characterization of Martha.

The British working class had rather less problem with Indians than people like Burnett did. They elected one to Parliament not long after My Secret Garden was published:

https://en.m.wikipedia.org/wiki/Shapurji_Saklatvala

I doubt Burnett was capable of imagining Martha voting for him (ok, she would first have had to imagine women getting the vote).