Superman is vulnerable to magic in the same way you or I would be.
So Mjolnir has the following powers (taken from Wikipedia so should be fairly correct)
Mjolnir can be used both offensively and defensively.
Described as impacting with sufficient force to "destroy mountains,"[36] with only primary adamantium proving too impervious.[37] Other offensive capabilities include creating vortices and forcefields (capable of containing an explosion that could potentially destroy a galaxy);[38] emitting mystical blasts of energy; controlling electromagnetism; molecular manipulation;[39] and generating the Geo-Blast (an energy wave that taps a planet's gravitational force),[40] Anti-Force (energy created to counter-act another force),[41] the thermo-blast which can even challenge such beings as Ego the living planet,[42] and god Blast (a blast that taps into Thor's life force).[43] The hammer can travel through planets to return to Thor(Thor Vol 2 #4).
There are also other several rarely used abilities. Mjolnir can track a person[44] and mystical items;[45] absorb energy, such as draining the Asgardian powers of the Wrecking Crew into the Wrecker;[46] or detect illusions, as Thor once commanded the hammer to strike the demonic Mephisto, who was hiding amongst false images of himself.[47] As a former religious relic, Mjolnir is lethal to undead, causing creatures such as vampires to burst into flame and crumble to dust.[48] Mjolnir also can project images, as Thor shows a glimpse of Asgard to fellow Avenger Iron Man.[49]
Mjolnir is simply > Superman and then you need to add Thor into the equation.