The issue is that there is somewhat of a no-limits fallacy in regards to Mad Jim, primarily because, despite the cosmic nature of his powers, he never was shown facing off against any of Marvel's usual cosmic tier characters (Galactus, Eternity, L.T., etc)
As such, the only hard limits we can exactly give to his powers is the absence of reality, and the Fury, a being inadvertently designed by another version of himself to adapt to his powers. Based on hyperbole, it's suggested that 616 MJJ is omniversal and not even universal nullification could stop him, but we never see him on that scale, nor do we see him face any omniversal entities or multiverse-level weapons (such as the UN or IG). So again, it's somewhat of a no limits fallacy.
Also none of this includes attempting to scale Marvel's multiverse with DC's, and Superman with Fury.