mattoid

One who exhibits symptoms of mental degeneration but is not positively insane; a semi-insane person; a crank: a term used by Lombroso to designate a semi-insane person whose ideas and aims (often of a literary or artistic character), while they may simulate those of talent and even of genius, are marked by radical absurdities which the patient is unable, through mental weakness, to perceive.

A person of congenitally abnormal mind bordering on insanity or degeneracy.

Displaying erratic behaviour

A person who displays such behaviour

Compound of genius and fool