Problematic doesn't mean that a thing needs revision; it just means that it needs contextualization or critique. You can enjoy a thing and still understand that it's an imperfect gem.

In terms of reading classics, it's up to the individual reader. It's valuable to know what the writer intended, and how their biases or upbringing may have affected them, and it's fine if someone isn't in a space to do that.

(For what it's worth, I think Mary's racism in Secret Garden is not meant to reflect well on her - it's there to highlight her bad attitude and arrogance before she learns to be better.)