In my scholarly opinion, A King in any right should and would be expected to uphold the highest possible level of reason which humanity allows
According to the apparent response from the administration was the reason for refusing the Siamese offer of War Elephants was because North America is not the natural habitat of Siamese elephents, and winters, particularly weren't ideal, Not to mention the torture and slog of the long sea journey in those days.
Therefore, a King of reason, which H.M. King Mongkut most presumably was, would not have taken offense.
In fact it is due to His Magesties King Mongkut, Rama IV, and his kin and processor, King Rama V, Chulalongkorns' 'levels of reason and communication intents' that Siam was the first of all the nations that the United States signed trade treaty with.