Mothra’s death in Godzilla: King of the Monsters was never meant to be permanent. A newspaper article in the film’s credits reveals that an expedition to Infant Island discovered a giant egg, implying Mothra laid it before aiding Godzilla in Boston. This confirms her ability to return at any time in the Monsterverse.
Director Michael Dougherty explained that Mothra embodies the cycle of birth, life, death, and rebirth. Her memories transfer to her offspring, ensuring she never truly dies. As long as she leaves behind an egg, Mothra can always be reborn.