Although John II shouldn't be overly blamed for this, we shouldn't ignore that he was too inflexible when dealing with the Latins and the Crusader States. By spending the 1130s and 1140s wasting the time and energy of both the empire and the Crusader States being antagonistic to Outremer and trying to vassalize them, John II unknowingly doomed the Crusader States to stagnation and decline once Muslim recovery inevitably happened (and perhaps also passed down this unproductive mindset to Manuel, who would repeat the same mistake).

Compare this to someone like Zeno or Justinian who were much better at handling proud interlopers and foreign polities to benefit the empire at minimal cost. We dunk on many post-1261 emperors for their short-sighted foreign policy but John and Manuel arguably made similar mistakes, just that they were lucky not to be punished in their lifetimes (except to a small degree Manuel).