The Superhero Database Classification number, or SHDB Class, is a number that represents the overall 'power' of a character. All traits of a character are used for calculating the Classification.
What it DOESN'T mean
This doesn't mean that a higher class would always beat a lower class character. But the bigger the difference in Class is, the more obvious it is who'll win in a fight.
How is this calculated
( INT^1.3 + (STR*0.5 )^2 + (SPE*0.5)^2 + DUR^1.6 + (POW + (SPS*SPL))^2 + COM^1.8 ) ^ TIER
Super Power Score and Level
Every Super Power has a score (SPS) that is used to calculate the Class. Each Super Power also has 3 levels (SPL). The level is set when connecting that Super Power to a character. The level determines the final score, of the Super Power, being used in the calculation.
Mothra's only advantage here is maneuverability. Her main defense, the reflective scales, only block one of Hedorah's attacks, while his acid spit will burn right through her (this spit melted Godzilla's eye off and melted his arm down to the bone). Mothra also has no way of permanently harming Hedorah either, as he requires a ludicrously high amount of heat to permanently damage. Adding onto this that any time Mothra tries to make contact with Hedorah she'll be losing a part of herself and we have game.
Could Mothra win? In specific circumstances, like if they were at a volcano. But in the vast majority of cases, she's going to lose.