Not true. Juana did inherit Aragon and was Queen of Aragon. But ,like in Castile , she was proclaimed Queen together with her son. From 1516 to her death, all laws and documents were proclaimed in their names, as Queen and King, her name being always before his. http://eurulers.altervista.org/spain.html
But, as you say, it is true she never ruled in Aragon, like she did not rule in Castile, after being declare insane.
Who were some notable European royals who were in line to inherit thrones, or even succeeded -- only to either be deposed or forced to renounce their succession rights because of mental incapacity?
Other than the famous mad kings of Bavaria (i.e. Ludwig II and his brother, Otto, who nominally succeeded him), some examples coming to mind would be Czar Ivan V of Russia (the elder half-brother of Peter the Great), Prince George of Serbia (eldest son of King Peter I Karageorgevich, older brother of King Alexander I), and Emperor Ferdinand I of Austria.
Juana la Loca of Castile might count, since she was allowed to briefly rule the kingdom she inherited from her mother, Queen Isabel I. But between them, her father, husband, and son eventually took over. And, of course, she was not allowed to inherit -- much less rule -- her father's kingdom of Aragon. Instead (I believe), she got bypassed in the succession in favor of her son, Holy Roman Emperor Charles V.
I'm sure there are many more cases. Can anybody name them?