McCoy sure sounds extra cantankerous in that scene. But given the fact that he was in a really bad mood when he beamed aboard, essentially being impressed into service by Kirk and Starfleet, I figure he was just looking for ANYTHING to complain about.
He THOUGHT he was impressed, although that was a bit of intended exaggeration of the word. "Your revered Admiral Nogura invoked a little-known, seldom-used reserve activation clause. In simple language, they drafted me!"
Using the word thing could suggest a belief that the referred-to object might cause problems. I always thought that that prejudice was what McCoy was objecting to. Also, cf The Thing from Another World.
"Bones, there's a cosmic muffin out there." "Why is anything we don't understand always called an edible foodstuff?"
One sense of "thing" is to express disapproval or contempt for something: "it's some Vulcan thing", the tone of which is what I suspect Bones was referring to.
I love how Kirk (in my view) knows that McCoy is in one of his moods, and doesn't even acknowledge the question.
Reading the script, I see the conversation as the banter between two friends. I see McCoy venting his frustration at someone he trusts and respects. I see Kirk knowing when to respond and when to let his friend vent. I wish that there been more revealed on what McCoy was doing before he was reactivated to explain his frustration.
In an earlier draft of the script, it's revealed that McCoy went into veterinary medicine. He said his patients didn't complain so much. I don't know how long that lasted in subsequent drafts. Also, in Roddenberry's novelization of the TMP, I think he mentions that Bones has been studying Fabrini medicine.