Green’s Dictionary of Slang

(US) a remarkable person or thing.

1834		Bastinado 49: Beauty Billy had been a great man in his day — that is, a great man on a small scale, and delivered himself as fast as he conceived. He was a thorough bred jim-crack, at beating the devil round the stump [DARE].
1937		AS XII 239/1: ‘He’s a jimcracker!’ is frequent in Minnesota to indicate that the person commented upon has exceptional skill; it is especially applied to persons excelling in games.
1961		R. Serling ‘Mr Dingle’ in More Stories from the Twilight Zone 36: It’s a handy-dandy, jim-cracker; A-one piece of merchandise.
1970		in DARE III 134/2: (‘In the very best condition’ ‘His farm is a ——’) [...] A jimcracky.