Whilst definitely the weakest in the trilogy, the original Mother is still an excellent game.
It is hilarious. The humour that can be found in the other two Mother games is present here making this feel like something truly unique for the NES. There aren't many games on the NES that will make you laugh as consistently as the original Mother, and this comedy is present throughout a pretty great story.
Whilst you can definitely start with any game in the series, the original Mother's story gives some more info on the origin of Giygas which makes him much more interesting in the sequel Mother 2 / Earthbound. Of course, most people didn't experience this additional backstory until after they played the sequel, as the original Mother took ages to get to the west.
Whilst a fully translated version of the game was made it was never released outside of Japan until the Wii U where the game was released as Earthbound Beginnings. Playing the original Mother is honestly one of the best reasons to still own a Wii U. The sooner it comes to the Nintendo Switch Online service the better.