Ethnic Scrappy: Diamond treats Wang like this. So does Twain. Wang treats his own son like this, too.
Shallow Parody: The way Miss Marple and Hercule Poirot are parodied has completely nothing to do with these characters. Miss Marple was prone to long-winding stories about her home village and was a mild-mannered sweet old lady. Poirot was very composed and precise and, most of all, very polite to everyone he met, he wouldn't dream of screaming at people. The characters in the film are pretty much their opposites, which would work. (Sam Diamond is played like a cowardly closeted gay, unlike Sam Spade.) The film also misses the point of Nick Charles. Dick Charleston is portrayed as "enormously well bred" and sophisticated. Nick's wife was classy, but Nick himself was a streetwise New York flatfoot, and one of the series' Running Gags was how little he tried to fit in as a socialite.
It wouldn't be too hard to assume William Powell's Nick was posh, given his snappy banter with Myrna Loy and penchant for really nice suits.
WTH, Casting Agency?: Truman Capote was not an actor and it shows. His performance is often criticized as being far too over-the-top even for a silly comedy such as this.