Taking issue with "Make America Great Again" to imply that the country was never great in the first plae is absurd. America is a great country, look at their record of democracy, of free speech, of civil rights, technological progression, innovation, cultural enhancement and so on. People only bring up that point so they can reply with pictures of slaves and think that they're making a noble point but they're not, they're making a point which is so selective, it has no merit.
The fact is, every country has a shady history. If you live in a country with a spotless record, you live in a recently colonised nation which essentially has no history, but if you live anywhere that has been historically significant, then your nation will have a imperfect past. That's because humankind is imperfect, there has never been a perfect nation, nor will there ever be. Britain has been a great country but it doesn't make the treatment of the Irish any better. France has been a great country but it doesn't excuse the Napoleonic wars. Germany has been a great country yet even in modern history, they have supported the Nazi party. Australia classed it's black people as "fauna" until the 1960s, Switzerland only made homosexuality legal in recent history, Italy has a woman alive today who was the first to publicly refuse to marry her rapist. And these are first world, 'civilised' nations, but if you go around the world today, you'll still find many countries that are racist, homophobic or misogynist by law.
These arguments go nowhere. It's not saying anything other than "in the past, your country was wrong"...well, yeah, and so was every other civilisation worth an ounce. Every country had racism, every country had homophobia, every empire stole resources, every prominent country took slaves...and that's just what history is, it's a frequently shameful thing to look back at.
The idea that "Make America Great Again" is therefore an invalid comment is nonsense because whilst there obviously were regrettable things in America's history, that negative aspect doesn't define them and nor should it. Their motor industry, sending man to the moon, developing vaccines, Hollywood, the music industry and all these other things are why America was - and still is - a "great" country. It can be better but it's not going to happen when people only focus on the bad.