there were definitely "mad" kings in the middle ages. King Charles VI of France was in fact known as "Charles the Mad".

in August 1392 en route to Brittany with his army in the forest of Le Mans, Charles suddenly went mad and slew four knights and almost killed his brother, Louis of Orléans.

From then on, Charles' bouts of insanity became more frequent and of longer duration. During these attacks, he had delusions, believing he was made of glass or denying he had a wife and children. He could also attack servants or run until exhaustion, wailing that he was threatened by his enemies. Between crises, there were intervals of months during which Charles was relatively sane. However, unable to concentrate or make decisions, political power was taken away from him by the princes of the blood, which would cause much chaos and conflict in France.

Charles VI's grandson, King Henry VI of England also suffered from mental breakdowns and bouts of insanity, including one severe instance in 1453 that lasted for well over a year.

many other European monarchs have suffered from various mental afflictions. Queen Joanna of Castile was deemed mentally ill after the death of her husband King Philip I and was afterwards known as "Joanna the Mad". another later Spanish monarch, Charles II, "The Bewitched" suffered from severe mental and physical handicaps, the unfortunate result of the notorious inbreeding of the Habsburgs.

a quick Google results in a Wikipedia list of mentally ill monarchs which lists many more than these few examples I mentioned.