Well, Northstar's personal take on Nemesis' face was just that: his personal take. The lifeless eyes, the burden she shoulders, perhaps Clayton simply isn't capable of conveying something so dark in his art. But I don't want to speak for him or anything.

As for being long-lived but not immortal, well, look at the Eternals, or the Asgardians. Many of them are youthful in appearance, but they aren't immortal.