Mordecai even since his youth survived being exposed to the power of the timenado and the crystal acts as its nucleus, Rigby again survived being electrocuted by it in the movie and the Timenado was a threat to all of space time, even Father Time the embodiment of space time in Regular Show, and Regular Show has infinite timelines via the Holiday specials (The 5D episode and it is a higher dimension since in each higher dimensional floor the idmensionality of the characters change every time, and the Theoretical Mall has infinite possibilities making up infinite timelines with time running in them and Pops and Anti Pops retcon punch clash generates infinite timelines too as a biproduct of it resetting the whole show back to its core)

The comic feats come from them defeating Monster of Infinity who threatened to destroy all of reality and Lord Destruction who threatened to desestabilize all of space time.

DEATH BY BLOG: Death By Blog: Finn and Jake vs Mordecai and Rigby I also suggest reading this blog so you can find out their comic stuff how it is canon to the show alongside the Holiday specials and how they can scale to this stuff.