Is Lethality bad against jungle monsters?

Minions and monsters actually do gain levels as the game progresses.

Minions gain levels every 90 seconds (every third wave)

Monsters gain levels based on the time at which they spawn. They do not gain level while they are alive.

I remember seeing a table that showed the level and their respective gold and experience based on time. I don't recall if it showed stats like health, armor, magic resistance, and damage.

I was just trying to find it again on the LoLWiki, but am having trouble.

What do you mean by "bad?" Sorry, but it's not clear what you are even asking.

As for how it works, it works as it's supposed to. Armor Pen is Armor Pen; scaling flat armor penetration (Lethality) doesn't function any differently.