I think that people are too quick to say "Well, all the emperors with bad reputations were the ones who antagonised the senate, clearly it was just senatorial historians setting out to blacken their names." A man who's enough of a megalomaniac to have sex with and then murder his mother or declare war on the sea is also likely to be a real dick to those beneath him; conversely, a moderate and lawful ruler is also likely to respect his subjects' prerogatives. So I think we should expect a high level of correlation between "Total madman" and "Doesn't get on well with the senate", and between "Good ruler" and "Does get on well with the senate".
Plus, not every emperor whom the senate didn't like was tarred with as black a brush as Caligula or Nero. Claudius and Domitian, for example, had a troubled relationship with the senatorial class, but whilst they're portrayed negatively, I don't think anybody's ever claimed that Claudius had sex with his sister and then cut her open to eat his unborn baby, as Caligula is supposed to have done.
For example, the horse appointed as consul was not real but an insult to the Senate, the guys writing the history books.
Well, some modern historians have suggested that it was an insult. That doesn't necessarily mean that they're correct.