cliff_marsland is making a
good point. Some nicknames gained currency and were generally used, and possibly were known, and perhaps tolerated, by the so-denotated emperors themselves (I suppose 'Caligula' might be one). Of these there are traces in the extant literary sources. But the really interesting nicknames would be those that were only used privately. behind the emperors' backs.... (was
Nero known as 'fatso', or some other emperor as 'the windbag' or 'the bungler', in some circles?). But then, what sources could we have on such as these? cliff rightly suggests
graffiti, but how could one know just to whom a particular graffito refers - dicey chronology or circumstantial evidence?