Boingboing.net might not be the most appropriate source for historical documents, but they share a picture from an article in the New York Times from Nov 22. 1922 with the following quote:
"Several reliable, well-informed sources confirmed the idea that
Hitler's anti-Semitism was not so genuine or violent as it sounded,
and that he was merely using anti-Semitic propaganda as a bait to
catch masses of followers and keep them aroused, enthusiastic, and in
line for the time when his organization is perfected and sufficiently
powerful to be employed effectively for political purposes."
The original article is available for NYT subscribers only on their website.
Alan Bullock (a British historian and contemporary of Hitler who later wrote a biography of Hitler) is quoted in an article by the German Newspaper "Die Zeit":
„Der Fehler, den wir alle begangen haben [..] war da wohl, dies für
reine Rhetorik zu halten, obwohl es ganz wörtlich gemeint war"
which translates to
"The mistake, made by all of us, [..] was to consider it (sc. Hitlers political program as
per "Mein Kampf") simply rhetoric, even though it was meant quite literally"
But that was said long after the fact.
I am still digging for German quotes, but so far it seems the only one who suggested to take Hitlers program not too seriously was Hitler himself when he promised to tone down and adhere strictly to the law in his bid for chancellorship.
Half an hour later and after digging trough the literature the closest German quote I can come up with is a editorial in the newspaper of the Centralverein deutscher Staatsbürger jüdischen Glaubens which commented after Hitler became Chancellor:
"Auch in dieser Zeit werden die deutschen Juden ihre Ruhe nicht
verlieren, die ihnen das Bewusstsein untrennbarer Verbundenheit mit
allem wirklich deutschen gibt"
"Even in these times, German Jews won't lose the peace and inseparable bond that they know exists between them and everything that is truly German
Even that this does not refer specifically to Hitler and sounds rather like a plea hoping that national socialist politics would not come to pass (quoted as per Friedländers "Das Dritte Reich und die Juden", translation by me).
All in all it seems that Adolf Hitler was taken pretty seriously in his home country.