Well, the 3 games have different Mother themes so I don't think the name is stupid...
In the original game (Mother for Famicom/NES) you have to save Mother Earth, you live in a town named Mother's Day, you have to defeat an evil alien using the lullaby that his surrogate Mother used to sing him when he was a baby...
The other games just have the Mother name because they are the sequel but they still having some Mother themes... I don't remember them exactly anyway...
In the second game you have to visit eight santuaries to get the power of "mother earth", I dont know why, since you just beat the final boss praying. The boss is the same from the first game, this time you beat it for real.
In the third game, two kids lose their mother...
The third game is the one most full of low kicks, since a lot of good characters die. this time the plot goes about waking a dragon to gets its power...
And yes, is a stupid name, cause if the game series havent becomed popular because of Internet and the smash bross games, would you buy a game called mother?