Just how stupid was Ar-Pharazon?

[Numenorean]

Firefoot

If Ar-Pharazon had paid attention to other's mistakes - pride, need to control - maybe he would not have made the same mistakes

Pharazon may not have viewed 'others mistakes' as we do. To him what you describe as pride and the need to control, were probably things he viewed as assets, such was the depth of his nations spiritual corruption.
Was Pharzon stupid? I don't think so. A supremely arrogant Warmonger, definately. To blame his decisions solely on stupidity would fail to take into account the steady rise of the Shadow in Númenor over the 1000 or so years prior to Pharazon.

From the reign of Tar-Atanamir onwards there was a growing mindset amongst the Númenorean elite that rejected the Valar and embraced instead their own petty fears and wishes regarding death - the Gift of Eru.
By the onset of Pharazons rule - with the exception of Tar-Palantir and the Lords of Andunië - the Númenoreans spiritual and moral beliefs had been relentlessly erroded for over a millenia, paving the way for Sauron the Deceiver to masterfully play on their hopes and half imagined fears, as only Sauron could.