Yes, a character can choose to skip added effects on a multi-target attack ability on a per-target basis. The rulebook states:

• "Added effects can be skipped, but the character must choose to do so before drawing an attack modifier card."
• "In the case of a skippable effect attached to a multi-target attack ability, they may skip the effect on an attack-by-attack basis, in each case choosing before drawing an attack modifier card, unless otherwise stated."
• "Conditional effects can also be skipped; the character is not required to pay the cost and, even if they do, they may still choose not to apply the effect."

This means for each target of a multi-target attack, the character decides separately whether to apply or skip the added or conditional effect, making the choice before drawing the attack modifier card for that target.

Source: page 22

Source: Core Rulebook (pp. 21, 22, 27, 37, 76)