It seems to me that MAGA was specifically intended not to have an explicit meaning, that it was intended to mean different things to different people. In a way it's a code phrase, intended to imply that Trump will reinstate the clarity of purpose, or the homogeneity of culture, or the greater relative power, or whatever, that the US was perceived to have in the 1950s and 1960s. And, IMO, with that clarity comes a simplicity of values and purpose that Americans didn't question 50 or 60 years ago. Communism and atheism were bad, the founding fathers were saint-like good, the US won WWII, US leadership in the world was dominant and unchallenged, the US was a dominant manufacturing power, caucasian Christians ran the country (with a few notable exceptions, who knew their places),and everyone else was a small minority. This is just my opinion, but I do think there are a lot of US citizens who long for the times when some or all of those things were true, and MAGA is code for any or all of this.
Frankly, I grew up in the 1960s, and I like the US of today a lot better than I did the US of 1965.