The problem with Canadians is that the Liberals are too smug, and that rubs people the wrong way. They do not hate Canada, they still like Canada, but people like Trudeau and Freeland are just plain annoying.
Remember what Reagan said about Trudeau the father? "Just a brat."
So these trolling comments and that social media picture was just to needle Canadian leaders, who are full of themselves.
Secondly, Canada being the 51st state, has been a long running joke for decades. Trump is repeating an old joke. He is old enough to remember those jokes.
It is kind of saying Canada is kind of like nothing, it is really not a country, more like just another US state. That viewpoint, will surely grate on those smug Canadian politicians who are full of themselves.