Mothra Vs. Godzilla (or as we know it in America and other points outside Japan -- Godzilla Vs. The Thing) offers up a tasty image of the classic Kaiju critter and establishes some of the tropes that will define the genre going forward in the Showa era and beyond. The first thing to say is that Mothra is the heroine of this epic, called upon by a modern world which has nuked her homeland, kidnapped her priestesses and progeny, and generally disrespected the people who worship her, to sail in and combat the deadly menace of the prehistoric engine of destruction Godzilla. And she does it.
Godzilla for his part is truly a monster, but in a different way. In his first film Godzilla was the embodiment of a nuclear weapon, wanton uncaring and unstoppable destruction and death. In his second outing Godzilla became a deadly and uncontrollable beast from a prehistoric time and impossible to control in a modern world. In the conflict with King Kong he became possessed of a personality and battled another monster for bragging rights and lost. Here in this latest face-off against Mothra he is downright malevolent. There is an evil intelligence evident in Godzilla as he wreaks havoc across the land once more. In some way his acts might be seen as somewhat accidental, but focusing on his muzzle shows intention behind wicked eyes. He is defeated and sent once more into the sea.
When he returns, he will become something else yet again, when an unwanted guest crashes the Kaiju party.