Thank you for this.
So we have 2 things going on here.
1. Alien is in the eye of the beholder. Drax, Groot, Superman, and the Asgardians are (AFAIK) 100% biologically natural representatives of their species. They are only "Alien" in the context of Earth, and D&D does not take place on Earth.
2. IIRC, the structure of your question was "In their media, are their any examples of mundane characters who can do superheroic things without relying on external power sources like superserums, mutant drinks, etc". The logic you've presented here is "if they can do things beyond the normal rules of the D&D game system, they cannot be mundane regardless of how they got there". It is perfectly self-defeating.
Of course no one can give you an example.of a mundane character who has superpowers without external help.
By your definition any character with superpowers cannot be mundane.