Yes, a Monster in Tokyo can heal through means other than rolling heal dice, specifically through Power cards.

This is supported by the following rule:

• On page 4, under the "heal" section, it states:
  "If you are outside of Tokyo, gain 1 [Life Point] for each [heal face] rolled.
  If you are in Tokyo, the [heal faces] you roll do not let you gain [Life Points] (you can only gain [Life Points] with Power cards)."

This clearly indicates that while rolling heal dice does not allow a Monster in Tokyo to heal, Power cards can allow a Monster in Tokyo to gain Life Points if the card effect allows it.

Source: page 4.

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