Earthbound is a really good video game but not a very good RPG, which kind of holds it back since it is, after all, an RPG.
The game has a really charming sense of humor and a really great localization, and aesthetically it's extremely unique and mostly well done.
But it's really straightforward as an RPG. There's really only one way to play through the game and no real decisions to be made with regards to equipment, (if it cost more at the store, it's stronger 95% of the time) party, (you never get more party members than you can use at once) or techniques. (There's only really magic and items. You learn new spells by leveling up and that's about it.) The only unique system is the rolling HP mechanic which frankly, very rarely matters.
Ultimately I think the game knows that it's not really anything special as an RPG and there's not much mandatory combat and very little of it is difficult. If you want to play an offbeat adventure game and don't mind being interrupted by a lukewarm RPG throughout then you could love Earthbound. If you want Dragon Quest with a humorous modern setting, this game actually isn't that, even though it sorta looks like it.
If you haven't played Earthbound or Mother 3, just play Mother 3 first and come back to Earthbound if Mother 3 made you cry.