Old-school Kaiju (Japanese for "strange beast") movies can be a ton of fun, with all the cheesy special effects and melodrama those types of films entail. Mothra is a perfect example, with some surprisingly great shots and a novel presentation of a monster whose only crime is being too protective of the subjects who worship her like a god.
The tagline even does a pretty decent job of describing the film, which puts it ahead of most of the films on the list: "Mightiest Monster in All Creation! Ravishing a Universe for Love!" Unfortunately, there are two big problems there. Number one, Mothra hangs out with Godzilla, so we know for a fact that she's not the "Mightiest Monster." She's probably the mightiest moth, or even the mightiest mother if we count her tiny worshippers as her children, but definitely not the mightiest monster.
Number two, "Ravishing" as a verb is a pretty far cry from what Mothra actually does. "Ravishing," especially in the older vernacular, has a lot more to do with rape or kidnapping than the kind of building-crashing action that Kaiju movies are known for. While a giant moth monster ravishing an entire universe seems like the kind of movie that would attract a particular fanbase of its own, it doesn't really describe Mothra.