Green’s Dictionary of Slang

considerable, substantial, huge; extreme.

1984		D. Jenkins Life Its Ownself 148: ‘Why is he fooling around with a sitcom?’ ‘Money, Biff.’ [...] ‘Mondo scratcho’ .
1985		Teenage Mutant Ninja Turtles (TV series) Whoa! Mondo nutsiness.
1999		T. Dorsey Florida Roadkill 150: They tore off two more mondo lines.