Mother 1/Earthbound Zero/Earthbound Beginnings is the best and the only truly great Mother game because it actually feels like a late 20th century road trip movie adapted into an RPG.
Earthbound/Mother 2 is way too goofy and is mostly a retelling of the first game without the Giygas backstory. And Mother 3 is a very shallow critique of capitalism, modernity, and western culture which goes against the premise of the first two games, and it might as well be a Final Fantasy game.
I think the series in general is overrated, but if one game is deserving of the hidden gem status, it would be the first one.