I generally like colour blind casting
'This actress (who happens to be black) really aced the audition for the role of Estella, she was by far the best we saw, we must have her' is one thing. 'We must cast a black actress as Estella' is another thing entirely.
Having Estella as slovenly and unkept (wearing expensive but dirty clothes, no servants to clean her clothes or ladies maid to do her hair etc) combined with the poor manners reads as prostitute not young lady.
And why would Pip (who would have seen plenty of prostitutes), think himself common and coarse in comparison?