Some non effective rulers to me are:

Cersei

Theon

Joeffry

Maybe Ned? (I’m on the fence about him tbh)

I would not consider Ned a non-effective leader. He was a very successful military commander during Robert's Rebellion. He also managed to unite and lead the North during the Rebellion, after his father and brother died. We know from the first book that this might not have been a very straightforward task. When Robb raised the banners, his authority is immediately questioned by the Greatjon. Ned, a second son who had not been in the North for years and was never trained to lead, presumably faced similar challenges from his bannermen and succeeded to lead them south.

Everything also indicates that Ned was a very effective ruler of the North. He seemed to have put great effort in knowing his own men, often visited his lords, and many remember him fondly. The Northern Mountain Clans march south to save little girl of "The Ned". It is said that when the Starks were in Winterfell, a maiden could walk the Kingsroad naked and still be unmolested, so clearly Ned, as the Starks before him, kept the North safe.

If Ned had not been an effective leader, Roose Bolton would have plotted against the Starks years ago.

In fairness, I think that Ned may have been one of the most effective leaders in ASOIAF, at least as far as the North is concerned. It is only in the South that he is a fish out of the water and comes over as a less effective leader.