Explanation: The Roman Emperor Caligula, sometimes said to be mad, once threatened (or joked, depending on who you ask) to make his horse consul, one of the two leaders of the Senate. Caligula was a tyrant, a dick, impulsive, and petty, but one can argue he wasn't mad. Just... well, a tyrant, a dick, impulsive, and petty. And with a cruel sense of humor.
The Roman Emperor Elagabalus had a much more discernable form of madness - youth mixed with ultimate power. Elagabalus was a teenager when they (as their gender identity is a matter of perpetual debate) were oh-so-wisely granted absolute autocratic power over a continents-spanning Empire by the political machinations of their grandmother. Two of their more notable offenses were proclaiming themselves to be the Syrian sun god, and having young women hitched to their chariot like horses to pull them around Rome, whipping them all the while.
While you can say teenagers fulfilling all their most wild teenage fantasies with no one to stop them isn't 'real' madness, it seems closer to it than flexing on Senators and daring them to defy you as a petty show of power!
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.
I love a member of the Julian dynasty thinking someone being disabled makes them not a threat. Like motherfucker you’re in the dynasty of the epileptic twink that destroyed the republic