X-Force 23 spoiler?

Not necessarily. The key distinction here is that Lil's bio-aura isn't impenetrable -- it makes HER indestructible. It's a kind of energy effect that permeates every aspect of her body. Even after removing a piece of her hair and throwing it at an opponent, the hair retained it's diamond-hard state, demonstrating that her indestructibility was a quality of her body, not a skin-tight field of some kind. Therefore, the better analogy is to compare Diamond Lil to Colossus old-school Rogue, instead of to Skids or Unus for instance. All living beings, even diamond hard ones, have a working metabolism and physiology that can be disrupted.

But if her bio-aura makes her indestructable, wouldn't Mortis still have to expend more energy to kill Lil than she would for someone of nomal durability?

I think I just don't understand Mortis's power. It's an energy attack that she shoot from her hand to disrupt the vital functions of hobos and cats, right? But Diamond Lil is physically impervious. So wouldn't his disruptive energy not be able to enter into Lil's body through her impenetrable exterior just as her body is unaffected by other energy attacks? It's not like Rogue's absorbtion power that works immediately upon physical contact. And wouldn't most rocks be organic anyway?

Mortis's power doesn't forcibly penetrate Lil's skin, and it doesn't need to. The energy contacts Lil's body, invulnerable or not, and induces the lethal change it creates. Lil would be equally vulnerable to being turned into gold, or Masque's flesh-shifting, or some other form of molecular rearranging power. Her bio-aura has no other effect besides making her superhumanly durable, and just being durable won't protect you from being altered at a cellular or molecular level.