This Wolfenstein: Youngblood game guide page explains the importance of skull icons above enemy heads and next to mission titles. Their presence is linked to RPG elements, which have been added to the new part of Wolfenstein.
In the case of opponents, skulls are associated with the enemy's level of experience. Skull icon means that enemy's experience level is more than 5 levels higher than that of a sister.
In short - it is best to leave your opponent alone and decide to escape or sneak past them. If you want to kill him (e.g. because this opponent is related with a specific mission), return to the same place after you have advanced enough times (location of your opponents does not change during repeated visits to the same districts).
In the case of opponents at a similar or lower level of experience, the game displays full information. Then, you can pre-learn how demanding (or not) the battle might be. In the example from our picture Supersoldat is at the 18th level of experience.
Skull icons for mission titles have the same meaning as for opponents. A skull-marked mission will be far too difficult for sisters until they have advanced enough. Starting it is not advisable, as you may have problems even with killing individual enemies, not to mention completing the mission objectives.
For other tasks and missions, the recommended level of experience is correctly displayed in the journal. It is best to deal with missions from the same or a lower level of experience.