Suetonius says some bad stuff, but no other historians repeat him. Usually, they use Suetonius and expand it. They didn't have any problem repeating Caligula stuff.
It's unfortunate that there is little we have, other than by Suetonius, of contemperory writings about Caligula. (But if you read the account of a Rabbi's (forget his name) visit to see him, the emperor's instability is apparent). We can compare what Suetonious says about Tiberius, for example, with Tacitus' description. Suetonius' story about the fisherman found by the emperor's guards near Capri, and having his skin rubbed raw, is not recorded by Tacitus and it seems a bit silly so we have reason to doubt it. They say Tacitus was inclined to damn Tiberius' memory, regarding him as another Domitian (I am not convinced) but he does include good things about him, for example the tax reliefs to towns which had suffered from earthquakes. I don't know anything that Suetonius writes about Tiberius contradicts Tacitus. I see Suetonius as a sensationalist, but I feel most of what he writes is believable.