A Comm for Historymemes

You may be thinking of a non-Emperor - Brutus, the founder of the Republic, pretended to be simple to avoid the last king of Rome, Tarquinius Superbus, from killing him. Brutus hid his competence until Tarquinius went too far, and then quite competently led a coup/popular revolution to oust him and replace the monarchy with a republic. 'Brutus' even means 'stupid' - and yet it became one of the most honored family names of Rome, lmao.

EDIT: Oh! Or you may be thinking of Claudius! Claudius was a scholar, but had a terrible stutter, a limp, and shook; he claimed to have played these up during the reign of Caligula to avoid Caligula seeing him as a threat. The Praetorians actually acclaimed him as Emperor after assassinating Caligula, though, despite Claudius thinking they were going to ice him too, lol.