The Mighty Thor remains among the mightiest of Marvel's cast of characters. The God of Thunder, thanks to the might of his legendary hammer, Mjolnir, controls the powers of thunder and lightning. However, from time to time, the legendary God has been deemed unworthy of the hammer, and thus, has been unable to lift it. This can and has happened for any number of reasons, from existential dread to arrogance to a single sentence ("Gore was right"). However, one event that seemed to very effectively class Thor as "unworthy" was becoming a flesh-eating zombie who feasted on the remains of mankind.
Or, at least, it appeared that the Marvel Zombie iteration of Thor was deemed unworthy. That is, until Marvel Zombies: Resurrection #1, where we see Thor wield his mighty hammer once again. Does this mean that Thor, even as a zombie who feasts on flesh, is still worthy of wielding Mjolnir? The Gods must be crazy.
Marvel Zombies: Resurrection
The series focuses on the Avengers and X-Men going to space after Captain Marvel sends them a very distorted message and an image of Galactus -- dead and floating in space. They go, only to find a community of inter-dimensional zombies have hollowed out Galactus, making it their home to travel throughout the cosmos. The helpless X-Men and Avengers are trapped by the super powered legion of zombies, who turn them into their own.
Among those to travel to Galactus is Thor, who is quite worthy of his hammer up until he gets bitten by Gladiator (off-panel), after which he throws himself through the Avengers' space ship, killing War Machine in the process. The zombies then bring Galactus to Earth, where he plummets through the atmosphere, ending the issue with the promise of even more carnage.
The Thor we see in the comic is just a freshly turned zombie, but the hammer doesn't react by immediately dropping to the ground, as one might expect. Rendered an undead horror, surely he would be stripped of worthiness. Thor doesn't do anything to become unworthy in the issues preceding Jane Foster's Mighty Thor. The very doubts and anxieties in his heart is enough to make him unworthy of wielding the hammer. So makes one wonder, how is a zombie able to hold the hammer? Wouldn't a zombie be corrupted immediately by thoughts of carnage and bloodshed?
The Original Marvel Zombies
This new Marvel Zombies story is obviously something of a relaunch tangentially connected to the original, but the original Marvel Zombies story seemed to indicate that, no, Thor is not worthy of wielding Mjolnir once he becomes a flesh eating pile of monster-flesh.
In the first Marvel Zombies miniseries, Thor, still lingering to the remnants of his old identity, creates a makeshift hammer made of a cinder-block and wood to use as a zombie. Still, considering the voracious need for flesh these zombies have, the hammer is mostly useless then, since most fights in that series are fought by the zombies diving on a living target and feasting.
This continues as the series goes on, indicating that, regardless of how long Thor is a zombie, he's never going to be worthy of Mjolnir. So what makes this new incarnation worthier?
The Strange Fanaticism of the Zombies
One odd element of the new zombies is how the zombification process has altered the zombies' personalities. They remain intelligent and able to control their powers, but their sheer zeal at making people "Come and see" seems to indicate an almost zeal to their actions. They want to destroy, but also want to share the pleasures of zombification to a new group of victims.
In this sense, the zombies, while cunning and manipulative (as indicated by how they got the Avengers and X-Men to come to their ship), aren't evil, per say. They could just be too outside the realm of logic to really understand the difference between right and wrong. As such, from Thor's perspective, he is still worthy, which would give power to the faith in himself. He isn't unworthy. He's just... differently worthy.
Mjolnir: In Space, Everyone is Worthy
The other possibility is that Thor only uses Mjolnir in space. Without gravity, it is possible for anyone to lift the hammer. Red Hulk famously used this loophole to beat Thor within an inch of his life with his own hammer.
The only real problem in this theory is that Thor's actions in the issue seem too deliberate. Not to mention, his momentum is enough to charge him through the Avengers' spaceship. If he were just drifting through space, it would seem impossible for him to build up the level of force necessary to punch a hole through a ship. That means he is more than likely wielding it like he always has.
The Other Possibility
The other possibility is that this alternate universe -- which seems like the 616 Universe in many ways -- follows slightly different rules that don't totally align with the laws of either Earth-616 or Earth-2149 (the original Marvel Zombies world).
Consider the Ultimate Universe. In Earth-1610, Magneto is able to lift Thor's hammer. Now, Mjolnir is still impossible to lift if you're not worthy and Thor is still the God of Thunder. Despite this, Magneto is able to manipulate the hammer with his powers. Magneto does this while reversing Earth's polarity, thus putting the world into an apocalyptic disaster. People who destroy ecosystems shouldn't exactly be worthy of wielding Thor's hammer, yet here he is, doing just that without a single thing hampering him.
Of course, this is all speculation. It's highly possible that the hammer itself is an afterthought by the artists and writers. The essence of the Marvel Zombies stories are to have fun with the imaginary possibility of the zombies going off to depopulate universes. It's probably a bad idea to think too hard about why Zombie Thor is wielding Mjolnir... but it sure is fun.