Unless One Is Constructing a Half-witty Insult. . .

. . . “not a wit” is. . . witless. “Not a whit” = “not a bit; not an iota; not the smallest part,” etc. “Not a wit” simply indicates someone without wit, a dullard, a dummy, a subliterate “Dunning-Krugerand” wannabe self-pub writer. *heh*