Probably as much as can be said about the hammer itself is that it's a big hammer made of some metal strong enough to endure use by the Hulk. It looked like the prop (whether CG or physical version) might have been designed to make the hammer head look like a repurposed piece of machinery (with a decorative meshed-teeth design stamped onto the sides).
I'm guessing the question got temporarily closed as "primarily opinion-based" because the particular metal or alloy of which the Hulk hammer is composed is not just unknown now, but will likely remain unknowable -- in-story analysis of that one-time Hulk weapon will surely never occur, and "word of God" from writers isn't likely on this, either. Adequate in-film explanation is given for Thor's effective use of it, however, without hypothesizing about the composition of the hammer itself:
Puny human that I am, swinging a large (but human-wieldable) mallet would let me (at the cost of being slower) hit a target with more built-up force I can manage with fists or something lighter, due to the additional momentum I can build up before it's through its swing. (Thor had some smaller mace of some sort earlier, but I don't think he actually got to land a hit with that.)
Additionally, as Richard C. commented, Thor had been holding back previously.
Lastly, IIRC, that was soon after he had the Odin vision, telling him Mjolnir had been just a conduit for power that was still his (a significant change from the comics). That revelation gave Thor a big boost; the power that he had previously been able to impart through Mjolnir clearly had some kinetic components (flight, Mjolnir smashing through rock, etc.), not just literal lightning, so with the return of conscious control of all that power, he arguably became physically stronger, not just able to call down lightning.
(The "conduit" stuff raises other questions that may not be satisfactorily answerable, as there may be a little bit of a contradiction with the first Thor movie, with its whole "I now take from you your power!" and "Whosoever holds this hammer, if he be worthy, shall possess the power of Thor" incantations. e.g.: Did Odin ever truly remove Thor's power in the first movie, or just jumper cable it to the hammer, so that anyone else worthy that managed to wield it would have been leaching from Thor via the hammer? In Ragnarok, if Thor was being hobbled by a mistaken belief that without Mjolnir he was stripped of the associated power, should he have once again been as weakened as he was in the first movie?)