People keep bringing up the odinforce and matter manipulation. Where was matter manipulation or any of that ever used? Thor did nothing but swing the hammer at cap's shield.
Thor's hammer, wielded by regular Thor, has dented true adamantium before with a swing.
Odin powered Thor is stronger than Thor. A stronger Thor would swing the hammer harder. Thus, he would hit harder with mjolnir.
We know mjolnir's metal is harder and tougher than captain america's shield.
Captain America has taken attacks from regular Thor because Thor holds back his power, but also because it is regular Thor.
With king Thor, king thor had the odinforce making him physically much stronger. So he was swinging the hammer harder. Since he swung the hammer harder, he swung it hard enough to dent/damage Captain America's shield. That is all there is to it. The odinforce doesn't just give you magical abilities. it makes you stronger, and tougher. Thor's increase in physical strength allowed him to swing the hammer hard enough to damage captain america's shield.
Matter manipulation or magical abilities had nothing to do with it. Magical abilities are shown and mentioned on panel when they happen or occur. This was simply thor swinging a hammer as he usually does. Except he was just swinging it much harder than usual this time.
Also he didn't just dent it. He dented it with a blow, then later on, he hammered the dent out of the shield, by striking it with his hammer. If thor's hammer was equal in durability to captain america's shield, both the shield and the hammer would have been dented. If Thor’s hammer was lower in toughness than Captain America's shield, the shield would not have been dented, but Thor's hammer would have dented. However, what we saw was that Thor’s hammer dented the shield, whilst Thor's hammer itself took zero damage. Which means, conclusively, that Thor's hammer is tougher and more durable than captain america's shield. Normal thor doesn't swing the hammer with enough force to dent Captain America's shield. Odin powered Thor has the strength to swing the hammer hard enough to dent Captain America's shield.