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.
Melkor found himself less powerful so it can be concluded that he weakened! So Melkor became mortal.
The Oblivion on the other hand if exist in Tolkien's Legendarium, the Oblivion would be Primordial Darkness or Absolute Darkness or Absolute Nothingness but to a very terrifying degree because he is the cycle of the beginning and the end of Everything. He can devour Ea like a candy.
For example, look at the Chaos King. He is nothing compared to the Oblivion but that creature was able to destroy almost 100% of the Marvel Multiverse which to dwarfs Ea so the comparison will not be very important, and the Chaos King was stated to be on par with Multi-Eternity and even made Multi-Eternity fear, something that Melkor would only dream of during his lifetime.