No one is. It's a running joke that the media and the democrats don't get.

If he had kept it in that meeting, it would have been just a joke. After all, in that context it was just part of the banter of the discussion.

But he's been constantly bringing it up over and over again, in policy contexts (like the tariffs and the upcoming trade war). Which takes it out of the in-the-moment "joke" category into the "deliberate choice" category.

Plus, "jokes" are real. In the sense that they can be a deliberate tool of diplomacy and politics. If Trump is constantly and repeatedly making a joke about Canada that is demeaning to Canada, it is a very real dominance play to emphasize publicly and irrefutably that the U.S. regards Canada as a nation not deserving the baseline respect accorded another sovereign country. Again, the "not even a real country anyway" joke from South Park, but reversed. In South Park, the joke was that Canada is very much a real country and only the yokels in the U.S. don't realize it; here, Trump is using the "joke" to hammer home that even though Canada is a very real country that Trump does not have to treat them like one. He doesn't have to accord their head of state with the respect due a head of state or use his title, he can constantly reference them losing their sovereignty (and their legislative authority and control over their own border and their military, etc.) to become a subordinate unit of government as an 'improvement,' etc.

Trump's always liked to use humor as a way of demeaning or diminishing his opponents, because it gives plausible deniability. It lets you engage in very deliberate insults of another party - but if it's pointed out that you're being insulting, you can deflect that it's just a "running joke" and that it's other people's fault if they can't handle it. This lets Trump continually label Canada a weak and subordinate nation without coming out and calling them a weak and subordinate nation, and that's a deliberate choice he's making to do that to a friend and ally.