it's not just that Mew is THE very first pokemon to ever exist, and also THE common ancestor between every single species, but there's also the creation of Mewtwo and the Cinnabar Island diaries that go about the discovery of Mew themselves as well as Mewtwo.
and then there's Faraway Island. Probably one of my favorite places in the Pokémon World lore-wise, because of all that is implied. The fact that there's a sign at all, shows that Mew could have been put into this island by a human, maybe someone involved in the creation of Mewtwo who decided to seal Mew away so humanity wouldn't repeat their mistake and leave Mew alone? Or did Mew stumble into this island on their own somehow and the sign was already there?
and the sign itself, while mostly faded by time, still has enough to understand it. "Those whose memories fade seek to carve them in their hearts. All dreams are but another reality. Never forget."
One does not know the actual meaning of the sign, is it a warning that following one's dream blindly may lead to something akin to the creation of Mewtwo? is it just a way to say to follow one's dream? Does it even have any relation to Mew as a whole? I don't know but I love the fact it's there.
Like, dang, story-wise I adore what has been made with Mew.