Suetonius is a difficult source. He had access to the Julian archives and can sometimes produce evidence drawn from wills, letters, and other sorts of documents, but he's also a terrible gossip. How did he know what kinds of sexual exploits Tiberius got up to on Capri? He doesn't mention any source for that.

He probably read Tacitus, who claimed that while Livia lived she controlled him. Women being active in politics was always problematic for Romans. They could advise in private, but for women to be politically active was trouble - see the Triumvirs' response to Hortensia's protest of taxes in the Forum. I think the source for that is in Appian. You can find similar complaints about Claudius being under the thumb of his wives and freedmen.

If you want a positive ancient view of Tiberius, look at Velleius Paterculus. He knew Tiberius and served under him in Germany. He gives a very positive impression of the guy. He's writing during Tiberius' reign and is generally taken as a shill for the regime, but I don't think that's fair. That privileges the later sources, Tacitus and Suetonius, against the closest witness. He seems like a sharp observer of politics to me (for instance, his judgment of the Gracchi seems balanced compared to other ancient treatments). Part of the reason he's discounted I think, is that there's not a good edition or commentary out there. Woodman's commentary isn't even treating the whole work.

What historians are you reading? They should tell you themselves why they think Tiberius was a competent emperor. I don't know what to say without knowing the historians in question.