Green’s Dictionary of Slang

(Aus.) one who has become insane.

1943		Sydney Morn. Herald 29 Sept. 9/3: Edwin Finn gave a perfect picture of what troops in New Guinea call a ‘troppo’.
1985		‘Thommo’ Dict. Aus. Swearing & Sex Sayings 132: TROPPO — A person who gives the impression of being either very stupid, crazy, or having a screw loose.
2018		G. Gilmore Class Act [ebook] ‘Every time there’s a murder, some troppo turns up saying they saw the whole thing’.