Thor's hammer... doesn't weigh that much. According to MARVEL it's 42.3 lbs (19.19 kg) which is still a lot to hold... especially one handed. But the reason the hammer can't be lifted isn't due to weight but because it takes someone who is worthy to wield it.
It's a common misconception that Thor's hammer was forged from a dying star when it was actually forged in one. But let's say it was made out of a dying star, specifically the densest in the known Universe... a neutron star.
Once a massive star dies and goes supernova, the core may collapse to such a degree that the protons and electrons smash together to form neutrons, giving us a celestial body that can contain two times the mass of the Sun within an object that is only 20 kilometres in diameter. The neutron star's density varies from below 1.109 kg/m3 in the crust — increasing with depth — to above 6.1017 or 8.1017 kg/m3 deeper inside (denser than an atomic nucleus). A sugar cube sized amount of neutron star material has the same mass as all of humanity!
Taking the dimensions of Thor's hammer, and assuming it's made entirely out of that material, it would weigh over 5 quadrillion kilograms. So now you have 5 trillion metric tons, about 97 million Titanics, condensed into the size of an American football.
Let's say you and I are having a conversation, I'm holding my hammer, and I accidentally drop it - BOOM! Being alive is no longer a thing you do. In fact, a good portion of humanity has stopped doing the whole living thing. The Tsar Bomb, at 50 megatons, was the largest nuclear weapon ever detonated. Thor's hammer hitting the floor would be 1.3 MILLION times that. The closest comparison would be the Chicxulub Asteroid that's thought to have wiped out the dinosaurs. When the asteroid impacted, debris was ejected out of our atmosphere and upon re-entry, it grew so hot that it glowed white and rained down globally, setting whatever could be lit on fire... on fire. Earthquakes shook the ground, mega tsunamis crashed into land masses, volcanos erupted spraying ash into the air combining with the dust and debris to cover the surface of the Earth and fill the atmosphere, blocking out the Sun.
Since the hammer is so insanely dense and covers such a small area, if you put it on the ground it would drill through the Earth until hitting the core. So it's best not to touch anything with Thor's hammer.
However, it wouldn't even have to touch you to horribly kill you. Newton's Law of Gravity states that any two bodies with mass will attract each other, and we happen to have an object with an incredible mass, squeezed into an incredibly dense package. If you were 30 metres away, you would be pulled towards it at almost 335 m/s2, which is close to the speed of sound. At that point, things get really messy because parts of your body closer to the hammer will experience a stronger gravitational pull than parts that are further away and that difference near the hammer will be so dramatic that your body will ripped apart by them, also known as spaghettification.
If Mjölnir were made of a dying star, it'd be devastating. It would also make Thor one of the strongest super heroes ever. And before we say, "but Christophe, it's magic" remember what Thor said in the first film, "Your ancestors called it magic... but you call it science. I come from a land where they are one and the same." Or in the words of author Arthur C. Clarke "Any sufficiently advanced technology is indistinguishable from magic."