Don't look at any spoilers; don't assume this game is GOAT (heh heh). Your best bet is to just jump in with no expectations, though it may be a bit late to do so. Imo the game alone is worth it for the soundtrack, TobyFox did a great job on that. I enjoyed Undertale, but you're not really playing for the gameplay, it's all for the characters and story.
Basically this is statement I agree with, I played the game. No I wasn't impressed with the gameplay. Okay to be fair, yes I appreciate the battle system since it reminds me of Mario & Luigi, how the player can play game on any level without having to level up since yo can defend yourself in battle (meaning you can avoid taking damage if you know how) this means you aren't forced to grind levels like other games where you "Have to fight to level up to be strong enough to survive specific hits or damage a boss" (Xenoblade chronicles comes to mind) No rpg game should ever follow the battle system of XENOBLADE CHRONICLES, please be more like Mario & Luigi games devs.
Other than that, I like the music, the interest is the story and characters, is called UNDERTALE, the game focus is to tell a story, like CAVE STORY, the focus is to tell a story. Please keep your expectations of "gameplay" to a minimum if you wish to enjoy it better.
Though it is to say I also played earthbound, very late in life (Literally last year) and was also unimpressive after hearing great things about it. I DIDN'T EVEN KNOW ABOUT THIS GAME. The game was for SNES, at the time we was busy playing good ones like Final fantasy VI, Chrono Trigger, Super mario RPG, Secret of Mana, and Breath of fire to name a few. Is one of those games when you play super smash bros, unlock some characters then be like "Ness? Who the heck is that, Some boy with baseball cap, yo-yo and psychic powers?" Oh is from a game we never had. (same story with marth and roy in smash bros melee)