A Monster in Tokyo can only Yield when it loses life from a smash attack (i.e., from dice rolled by another Monster). If a defensive Power card prevents the Monster from actually losing any life, then it has not "lost" life from the attack and therefore cannot Yield.

This is supported by the following rules:

• "You can only Yield when you lose from Faces for dices (King of tokyo) (2 kinds of dice, black dice and green dice) rolled by a Monster." (page 5)
• "Yield: a Monster may Yield Tokyo only after being wounded." (page 6)
• "Wound: a Monster wounds another Monster when it makes them lose one or more with their Faces for dices (King of tokyo) (2 kinds of dice, black dice and green dice) during the Resolve Dice step." (page 6)

If no life is lost (i.e., the Monster is not wounded), the Monster cannot Yield.

Source: pages 5 and 6.

Source: Core Rulebook (pp. 3, 4, 5, 6)