Why does Voldemort look so disfigured?
It was because of creation of his horcruxes. His soul got mutilated because of it’s ripping so many times and thus Voldemort grew less human in appearance.
Why does Voldemort look like what he does?
Splitting his soul seven times to put into horcruxes left him less than human. He became a monster. This also had an effect on his appearance. As he lost a bit of his humanity each time he killed someone and split his soul in two to make a new horcrux, he also looked a bit less human and more like a monster.