That is an interesting question. I think Katzman helped making the show into the success it was. I remember reading an interview once where I think it was Deborah Shelton that said that Leonard Katzman had used his own marriage as inspiration for the development of JR/SE. If that is true he certainly succeeded in creating some sort of dysfunctional power couple that people enjoyed watching.
I think Dallas reached it's peak during the 82/83 and 83/84 seasons. Katzman certainly had something to do with it's success.
After that the decline began with the recasting of Miss Ellie, dream season and Bobby coming back in the shower, Pam and Donna leaving etc.
Yes I think most of us can agree that he did some mistakes during the latter seasons of Dallas. But at that point the show was already on a downward spiral.
I think the biggest mistake they made was not knowing when to let go. They should have known beforehand when it was the last season and then brought back the people who had been let go to give Dallas a proper send-off.
Or maybe even gone out while they were on top. But if they had done that the show would have been at least 4 seasons shorter, maybe even 6 and I did find some enjoyable bits in all seasons.
So I guess the place where the show should have ended was after the 89/90 season. Letting Lucy, Miss Ellie and Clayton go and killing off April on her honeymoon was the final straw. For a show that used to be about two families to suddenly only be the JR-Bobby and Cliff show just didn't work.
Although to be honest, the first time around when I watched Dallas I wanted to watch more after the 90/91 season as I totally loved James and Michelle.
Still over all I think he did more good than bad.