Since playing all the games I think "EarthBound" does fit each game in the series better. In the first and second games there's the obvious point that there are aliens Bound for Earth, and in the second there's the whole deal with Ness visiting the sanctuaries (locations around the world) so that he can really extend his OP-ness--his powers are Bound to the Earth. In Mother 3 there is no obvious connection to the title "EarthBound" and no aliens, but, there is the dragon sleeping under the Nowhere Islands, presumably literally pinned down by the needles... so the dragon itself is Bound beneath the Earth.
So I know there's, like, emotion and other pretentious crap in the title "Mother" but we've got what we've got and it'd be cool to be able to start calling M3 EarthBound 2 without people being like "wuwhat"