You are not. However, that is only the motivation for what he did. The ACTIONS that led to his downfall were dishonorable actions, which he took mainly for the reasons you ascribe to him.

There is nothing remotely honorable about his warning Cersei, or refraining from telling Robert right away. As Robert's friend, his duty was to let him know about the fraud his wife was perpetrating on him and that it had caused the death of their foster father. As a loyal subject of the Seven Kingdoms, it was his duty to uphold the law and report treason, rather than concealing it. As the Hand of the King, it was his duty to enforce the laws and protect the king and the rightful succession, and punish treason, not give the traitors a head start!

The issue was never Ned's stupidity, it was his reasoning and motivations. You can't call Ned stupid for his actions because he was not trying to do the smartest thing, he was trying to avoid unpleasantness (i.e. Robert trying to kill the Lannisters, etc). Instead he facilitated a war that killed members of practically every noble family in the Seven Kingdoms.

Ned did not do the honorable thing in refusing Renly's offer or suggestion that they kick off the anti-Lannister coup early, rather, once again, he quailed at the idea of upsetting a loved one. Disturb Robert's rest? Oh, too bad for Robert! He's the damn king! His reign appears to have been one long exercise in self-indulgence, he can endure a little upset to right the wrongs being done to his kingdom which might have been avoided had Robert been a more attentive ruler and husband, rather than merely concerned with stuffing his face and screwing anything with a hole in it. Granted, he was right not trust Renly's motivations, but he shouldn't have trusted Littlefinger either. Prudence IS one of the cardinal virtues after all, and there is nothing dishonorable about keeping your mouth shut until you are in a position to do something effective.