it feels do able depending on the right person, since undertale is a pixelated game with sprites it wont be to hard to edit them fat. a good example is the cave story + Sized made by a colaberation of Fawxen and a few of their friends to create cave story but with fat sprites.
well, here you have a sprite but it is from the combat mode, does this help you?.
But How similar do you want the style to the base game? And who would be made fatter?
basicly everyone since it just a sprite change and doesnt affect the actual gameplay, that why i reffered to the cave story mod someone made
I mean, I have no clue you might look at this but there is a Undertale Mod Tool That can edit the sprites, that is, if you want to do it?