The obviously foreign character of Poirot is definitely used to poke fun at Brits and their ~~racist~~ insular attitudes

Of course, there's also the episode where the disappeared Russian servant of a rich old lady is suspected of murdering her and being a Soviet agent. Poirot figures out this can't be the case because he finds a bunch of Orthodox religious paraphernalia and a picture of Nicholas II in her bedroom and it turns out that she is actually the Good Kind of Russian: an aristocrat who fled after her family lost their holdings to and is now forced to live under a false name and work as a servant. At the end she gets the old lady's inheritance and can resume her upper class lifestyle and hopes that someday Russia too can be truly free. Poirot's only love interest was also an exiled Russian noble who missed living in a mansion