I disagree. He was an atrocious playmaker. Even at long beach where he "flourished" he only averaged 1.3 assists for every TO (4.8/3.7). He could make some fancy passes, but he didn't have good vision, good timing and didn't set up his teammates.
He also wasn't a mediocre scorer. He was a terrible scorer. At MN, he was averaging more shots than points (that's atrocious). At LB and Iona, he improved to 1.17 points per shot. As a point of reference, EJ Stephens average 1.24 points/shot at Lafayette and Loewe averaged 1.36 points/shot at William and Mary. So he was an atrocious scorer at the Big 10 level and slightly less than average at a lesser level.
Even at Long Beach, he turned the ball over quite a bit, he didn't shoot well, and I doubt he played defense (I can't really speak to that).
Pitino's offense sucked. I hated it but ball dominant PGs could not have found an easier offense to flourish. I can't imagine how bad Washington would have been in an offense where he played off the ball. The only trait he had is that he could dribble. He couldn't make good decisions with the ball, but he could handle the ball. I don't think there are a lot of systems where a ball dominant guy who can't shoot, won't play defense, can't finish at the rim and turns the ball over will find success.
Pitino was absolutely lost as a coach, but Isaiah Washington would have even been worse with Ben Johnson. I don't think Johnson would have played him a minute.