Keep in mind that there are many factions of the Rebel Alliance, so I don’t see why she has to be everywhere Luke, Leia, Han, etc. is.
I have to point out that this is not true in the original movies. I suppose this might be suggested in the EU stuff, but in the original 1977 film, the Empire is attempting to find one hidden Rebel base and Darth Vader is confident that destroying that one base will result in the end of the Rebellion.
That’s true, but even the OT wasn’t always too consistent themselves. But yes, they definitely seem to have a Main Base that they move around. But it seems stupid of the to “put all their eggs in one basket” so to speak. Also I’m not denying that these are retcons, they obviously are, but that’s not exactly something that’s new to SW, and I’m simply saying that it’s not too hard to give reasonable excuses as to where Mon Mothma was during ANH and ESB when you take the PT, EU (both of them), and the new Disney canon into account. If you go from a OT purist perspective then yes, her sudden appearance in ROTJ s kinda weird, but in the whole franchise that’s developed since, there’s plenty to choose from.