Its unclear why Trump (his intentions are often hard to interpret) but more generally MAGA or Republicans are keen on adding Canada as a 51st state.

They're not. To the extent that Republicans have discussed it on the merits, rather than as a sideshow, they've acknowledged that adding the more-liberal-than-conservative Canada into our system would be bad (on the whole) for the GOP.

Trump uses a number of tools in his dealing with people and countries, two of which are dominance and hyperbole.

He places great importance on establishing dominance, which is why he goes in for weird handshakes and disparaging nicknames. Describing Canada as a mere "state" of the U.S., or suggesting that it would be a step up for them to give up their country in exchange for statehood, is another variant of that. Rather than ridiculing them with a silly nickname, he demeans Canada by continually suggesting they're not quite "sovereign nation" material. Or to quote South Park, "they're not even a real country anyway."

His real estate developer side comes through with his penchant for hyperbole - he likes big, exaggerated claims and statements and gestures. He has a predilection towards bombast and hype generally - so he's not going to just establish a dominant relationship with Canada, he's going to actually absorb them. He also has a fondness for moonshot proposals that will make him a significant figure in American history; for good or bad, he wants to be one of the Presidents that people will remember. No "Benjamin Harrison" will he be, that's for sure.

So while he'd love for it to actually happen, it accomplishes most of his goals just to keep harping on Canada being a state even if nothing ever comes of it.