Gotti definitely caused a lot of damage to the Gambinos, but i wouldn't say that he ruined the Mob. The Families had been in a steady decline thanks to RICO,more funding for law enforcement,the diminishing quality of the members themselves,and the abandonment of the "old school" ideals of loyalty and respect.
As far as the Gambinos go,Gotti absolutely was in over his head as Boss. He was Capo material at best.I would agree with you that the element that hurt the Gambinos the most was Gotti's runaway ego.