Entry from OED Online

Also ditzy. [Origin unknown; perh. corruption of DICTY a.] 

    a. = DICTY a. a, b. Also, fussy, intricate.
 
  1978 Detroit Free Press 5 Mar. (Spring Fashion Suppl.) 23/1 Forget about delicate chains, a ring on every finger, clanking bangles and ditsy earrings. 1979 OBERMAN & STECKLER I could have been Contender ii. 8 A ditsy manicurist who's crazy about cats and lives alone. 1981 N.Y. Times 3 Dec. C8/5 They'll cook and clean for a week before a party and worry over the ditsy little touches, the table, the flowers, the matching guest towels. 1985 Ibid. 29 Jan. C13/3 She also has a big repertory of comic voices, ranging from..a maternal croon to a ditsy English matron's stiff-upper-register.
 

    b. (Esp. of a woman) stupid, scatterbrained; cute.
 
  1980 Maledicta III. II. 245 In this new age of the New Woman, the only old-fashioned girl left may be the drag queen who apes a ditzy, decorative female largely obsolete, much as Mae West (darling of female impersonators) recalls the woman of a past time. 1981 Time 12 Jan. 45/1 Bob Newhart plays the President of the United States: Madeline Kahn is his dipso wife, Gilda Radner his ditsy daughter. 1982 Christian Science Monitor 26 Mar. 15/1 It is filled with charmingly ‘dotty’ and ‘ditsy’ people who engage in generally ‘gaga’ relationships. 1984 Washington Post 20 May H13/1 Willie Scott..is a ditsy blond who sings at a Shanghai nightclub. 1985 N.Y. Times 31 Jan. A22/2 According to a wholly unscientific sample, this decade's terms [for ‘dumb’] so far include, besides airhead, retard, ditsy and wifty. 1987 Los Angeles Times 7 June 16 What do father and daughter think of television's flashy detectives? Maddie and David on ‘Moonlighting’: ‘They never work. They're ditzy.’
 

    Hence (as a back-formation) ditz, one who is ‘ditsy’, scatterbrained, or cute.
 
  1984 N.Y. Times 9 Feb. 19/1 Miss Alexander's portrayals of Anna ranged from ‘a complete ditz’ to playing her ‘as Astarte’. 1985 Guardian 22 June 12/4 Meryl Streep is serious, Suzanne Somers isn't. That's the way they're seen...I don't think Miss Somers does ditsy tap dances when she gets home. I've been both. I used to be a ditz. Now I'm talented.