In the “response of the zhaporovian cossacks” the Cossacks refer to sultan mehmed IV as “the wheelwright” of macedonia. Given the nature of the text this seems to be meant as an insult, why would being referred to as a wheelwright in the 17th century be seen as an insult?

Well, unfortunately, the premise of your question is wrong. There is no historical evidence that the letter itself was written (at that point in time) or sent, certainly not in Turkish archives, and u/tanktango's answer here makes a good explanation of that: https://www.reddit.com/r/AskHistorians/comments/8qg1re/did_the_cossacks_really_write_this_letter_to_the/e0j7urp?utm_source=share&utm_medium=web2x

Unfortunately, this makes the answer to your question rather mundane. It's simply meant to be a mockery of the titles supposedly used by Sultan Mehmet IV, as any type of laborer's work would be considered an insult, which, again, is an open question as there is no evidence the Sultan sent such a letter. There are numerous reasons to think that he didn't, at least not in the form presented, such as the anachronistic and irregular use of "Babylon" as discussed here by u/atarole: https://www.reddit.com/r/AskHistorians/comments/54u3gi/in_the_letter_of_mehmed_iv_to_the_zaporozhian/d85i3hb?utm_source=share&utm_medium=web2x

The Sultan is also shown to be calling himself the "Son of Mohammad", which is not a title that is appropriate for Islam, but that's entering speculation territory on my part so I'll be ending the answer here. Hope this helps.