Why in the fuck other than for the plot would the epitaph of the first person on Mars include that he was named after his failson uncle who presumably died before he was born? By that point literally who cares? I got my middle name from my grandfather but that doesn't mean it's going on my tombstone.
I mean presumably Fry's brother mythologised him to his son, and it served as an inspiration. Maybe his failson uncle is what inspired him to seek out a career in space exploration, which is why he wanted the little blurb on the tombstone.
Or the writers needed it for the bit, who can say.