Outer Wilds is simply brilliant in many, many ways. I could write like 20 different essays about it. SPOILER obviously, don't read this if you ever intend on playing the game (you should).
Not just mortality, Outer Wilds does a great job at making you feel insignificant. Your spaceship is made of nothing but wood, duct tape and reckless optimism while you explore the remains of this ancient species that was so, so far ahead of you, you stumble into all their bizarre and amazing devices and constructions, but then at the end it turns out that even these great, highly advanced people that came before you died to something completely meaningless.
Their arrival in the solar system was merely a cosmic mistake, and their demise was no great downfall of an empire, it was a spontaneous, arbitrary mood of the universe. A meteor that just so happened to contain a big crystal filled with deadly gas flew too close to the sun and everyone just instantly and unceremoniously died. The Sun Station is an absolutely awe-inducing moment in the game for me, alongside the Ash Twin Project it was arguably the Nomai's magnum opus, and it couldn't even put a dent in the sun it was meant to destroy. The point being that you feel small next to the great Nomai, but the game also repeatedly shows that even the Nomai are nothing in the grand scheme of things.
But this is what makes Outer Wilds so special imo: You are small and insignificant, but that is okay. Terrible things happen that are completely out of your or anyone's control, but that is okay. The tone of the ending isn't ceremonious or particularly grand, nobody congratulates you for doing anything, it's just "Alright, we've had a good run but now it's time to say goodbye."
The message, to me, is basically "The meaning of life is to stop and smell the flowers", there is no greater purpose to anything but that's fine because the real purpose was the friends we made and the things we saw along the way.
Edit: I'm rereading this and it's kinda incoherent but that's because I'm not joking about the 20 essays I could write and trying to condense my thoughts about Outer Wilds into a comment appropriately sized for hexbear dot net is quite difficult.