No, there's no reason to believe he was. Of the many things Graves came under attack for his portrayal of Livia and of people's opinions of Livia was probably the most serious. In the introduction of one of the reprints Graves actually had to defend his assertion that Livia was scheming by going into Tacitus and pulling out relevant passages, and even then the best reason he could come up with to defend Claudius' vehement hatred of Livia in the book is that he had every reason to hate her so it makes sense in the story if he did. I, Claudius is wonderful, and it actually had a pretty important historical impact, since Graves was among the first to seriously suggest that Claudius was not the bumbling fool that Seneca paints him as, but his characterization of Livia is very unfair. Livia was hated by a lot of the Roman aristocracy, partly because she was a woman who was quite consciously allowed by Augustus a great deal of power (I find Graves' portrayal of Augustus as this fool oblivious to Livia's actions a little bit silly), but also because she had broken some pretty important social conventions to get to Augustus' side. After all, Livia divorced her previous husband, who was a devout Republican whose father died at Philippi, and suddenly changed sides after Philippi and ran off with Octavian--while she had a baby and was pregnant with another! Livia's sudden turnaround is extremely odd, since her husband was a partisan of Lucius Antony's and was forced to flee Italy during Octavian's proscriptions. The fact that Livia suddenly reversed her policy on Octavian, running away despite now being a mother as well, looked very very bad in the eyes of Roman aristocracy, who quite understandably thought that Livia was just trying to save her own ass. The alternative to an opportunistic Livia isn't very pretty either. Several of our sources note that Octavian was obsessed with Livia from the moment he saw her, which Graves mentions. Although slightly better than the idea that Livia just wanted to save her skin and so worked her way into Octavian's inner circle to do so, it still didn't reflect well on either Livia or Octavian that they were so immoderate as to suddenly divorce and essentially elope. Keep in mind that Octavian divorced Scribonia, whom he had only been married to a year--Dio reports that the divorce took place the same day that Julia was born. That didn't look real good, morally, and for much of the Roman elite it was made worse by the fact that divorcing Scribonia for Livia was a complete insult. Scribonia's marriage had been intended to calm the partisans of Sextus Pompey, to whom she was related, and keep the last of the Pompeians from causing trouble. Although Octavian's divorce certainly didn't single-handedly send Sextus Pompey into open revolt, it made affairs between them extremely tense, and certainly played some part in Pompey's continual attacks on Octavian. Now, personally I suggest that Octavian's marriage to Livia, which took place at the time when he pardoned Lucius Antony's followers, was an attempt to shore things up with Mark Antony. The marriage also coincides with the Pact of Brundisium, which renewed the Triumvirate and prevented open war from breaking out between the Triumvirs following Octavian's handling of the War of Perusia. A marriage to Livia, the mother of the children of one of the most prominent Antonians, would have been well met--Antony himself didn't really have any female relatives to marry off. Still, the Roman elite wasn't too happy about it, and even if my suggestion is true it probably didn't sit too well with them, since the Triumvirate was a serious limit on their privileges. Rumors flew around about Livia, but they mostly appear after her death, in Tacitus and Suetonius. Livy, Virgil, Horace--all of Livia's contemporaries have nothing but good things to say or imply about her (to the extent that they talk about her at all), even when they don't agree with Augustus. So I'm very much inclined to think that this is slander, although I rarely take Tacitus at face value (he's just got too much of an axe to grind, and it shows whenever he talks about the good ol' boys)

Most of this portrayal is an exaggeration of the depiction of Livia from Tacitus, where she is portrayed as both noble and intelligent, yet on the other-hand overly loyal to Tiberius, domineering of Augustus in his elder state, and most importantly, possibly culpable in the deaths of Augustus' grandsons etc.

I don't think there is any doubt she was a very intelligent woman, but even Tacitus, who has every reason to slander her, doesn't outright characterize her as the harpy she would later become in certain popular depictions.