I think Christie also sideswipe the casual racism of upper class (white) British people, too. Poirot is often the subject of zenophobia. His foreigness is pointed out in numerous "those people" and "what do you expect from foreigners" comments. Our hero may not rant and rave about the injustice of ridiculous comments made by small minded, high handed, unpleasant people, but we do witness the insults with him. Poirot is a stoic intellectual: his mustachioed upper lip may well be Belgian, but it is definitely stiff.

Also,as you say, the books are written of their time too. Christie was born a Victorian, after all.