I was very vocal about Rumpels’ weak healing in beta—as you say, a 5* average mana Heal should be better than a 4* average mana Heal, particularly because it’s uncertain. After the devs tweaked him, they lowered his Heal! Very odd choice, as no one had complained about that.
The Stars skill is awkwardly weak. Most other specials have a higher % boost/debuff, so if you fire Rumples while one of those is active, you undermine the earlier special. For this reason, I think Rumples is only useful for players with a fairly thin set of hero’s.