"Even the ice-hard heart of Hades melted." Is this a hyperbole?
No, because nothing is being exaggerated.
This is simply a simile (Hades's heart was as hard as ice) used to emphasize the heart melting ability of whatever melted his heart.
I think it is hyperbole because the very idea of Hades having a heart that can melt is exaggeration.
Let's talk this out a bit to why I think we have hyperbole here.
First off, Hades is the god of the dead and the underworld. It's Hades who collects each and every person at their appointed time and sends them to wander endlessly in the underworld. He ignores all pleadings from those dying, the cries and grief of family and loved ones - he is cold and callous and unmoved by anything.
So what would it take for the heart of Hades to melt? If even death can't move Hades to care about anything, what would it take? My answer is something so over the top that even death pales in comparison.