What's wrong with 'Make America Great Again'
1.) Geographical inaccuracy: 'America' is two entire continents, including the USA but also Brazil, Canada, Argentina, Nicaragua, Panama, Uruguay, the Falkland Islands, several others that most US Americans don't give a toss about, and Trump's particular bugbear Mexico.
2.) Implication that their country isn't great already
3.) Implication that it would be great if less people had prosperity and safety (no-one at all is buying "prosperity and safety for all")
4.) Implication that Trump supporters want to destroy all the good things that happened in their country over a recent period of time (such as the end of the Cold War, end of segregation, anything publicly funded etc.)
Also it's a slogan not a word... MAGA is sort of a word, though, an acronym. Or it would be if anyone could agree how to pronounce if anyone could agree on pronounciation (Mayger? Magger? Marger? Maggar?)
It's sort of like the next Democrat says they want to heal the USA, so uses the slogan 'Educating Conservatives'. Then when the conservatives get rightly pissed off, his supporters act all innocent and say "What's wrong with educating people?"