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.
Every superpower he came close to he can simply absorb / copy / mimicry, and then use it himself too.
That's how - over time - he acquired very big arsenal of powerful special abilities.
Even if only 1 ability by itself maybe wouldn't be good enough to be a danger for Dormmamu, it's the fact that Peter has got dozens of more overpowered hax AND - if Peter is anywhere near Dormmamu Peter could easily just mimicry Dormmamu's own abilities , too.
Peter is too powerful and wins.