A monster cannot yield Tokyo if they lose life due to a Power card effect. According to the rulebook:

• "A Monster can only Yield Tokyo when losing from Faces for dices (King of tokyo) (2 kinds of dice, black dice and green dice)." (page 4)
• "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)
• The glossary further clarifies: "Yield: a Monster may Yield Tokyo only after being wounded." and "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)

Losing life from a Power card effect is specifically stated as different from losing life from dice, and only the latter allows a monster to yield Tokyo.

Source: pages 4, 5, and 6.

Source: Core Rulebook (pp. 2, 3, 4, 5, 6)