Of American-English origin, the colloquial and usually derogatory noun bimbette is from:
– bimbo, denoting a young woman or teenage girl who is regarded as sexually attractive, but unintelligent or frivolous;
– the suffix -ette—it is unclear whether, here, this is a diminutive or a feminine suffix.

The earliest occurrences of the noun bimbette that I have found are as follows, in chronological order—the first two are from reviews by the U.S. film critic Richard Corliss (1944-2015):

1-: From the review of the U.S. thriller film Jaws 2 (1978)—review by Richard Corliss, published in New Times (New York City, New York, USA) of Monday 10^th July 1978:

The illustration for the Jaws 2 ad tells the story: The great white shark, with its machete-blade teeth and its dark eyes the size of bowling balls, shoots out of the water in pursuit of a smiling, water-skiing bimbette in matching golden hair and bikini.

This is the illustration for Jaws 2 Richard Corliss was referring to:

 

2-: From the review of the U.S. drama film Sophie’s Choice (1982)—review by Richard Corliss, published in Time (New York City, New York, USA) of Monday 13^th December 1982:

She is the secret heroine of Hollywood movies: the divine masochist, the superior woman battered by fate, society, ham-fisted men and her own acute facility for self-destruction. Serious actresses, itching to play something more demanding than bimbettes and stand-by wives, love divine masochist roles. They get to run through huge emotions, from innocence through every sordid experience, often embracing rarefied forms of madness and an early, spectacular death.

3-: From the review of the U.S. drama film Table for Five (1983)—review by Thomas Fox, published in The Commercial Appeal (Memphis, Tennessee, USA) of Friday 25^th March 1983:

Dad hasn’t reformed at all. He has reserved a table for five in the ship’s dining room, with the extra seat ready for the woman he will select from the passenger list. Tucking everyone into their beds, he goes off for an evening of slap and tickle with a fleshy-French bimbette (Marie Christine Barrault ¹) who has daughters of her own and a marriage in her past.

¹ Marie-Christine Barrault (born 1944) is a French actress.

4-: From Alan Thicke: Not Twisted Enough?, by Susan Stewart, published in the Philadelphia Daily News (Philadelphia, Pennsylvania, USA) of Monday 28^th November 1983:

Alan Thicke ² could be funnier. He could be more cynical, more twisted. He doesn’t want to. “I could sit with some third-rank hunk or bimbette from a canceled sitcom and tear them to shreds,” says Thicke, whose gentility can be witnessed nightly on his talk show “Thicke of the Night” (Channel 17 at 11:30 p.m.). “I would feel unclean if I scored at the expense of stupid guests.”

² Alan Thicke (1947-2016) was a Canadian actor, songwriter, and game and talk show host.

5-: From the review of the U.S. detective television series Riptide (January 1984-August 1986)—review by John Voorhees, published in The Seattle Times (Seattle, Washington, USA) of Friday 6^th January 1984:

It’s really about male bonding. There’s so much good-natured badinage between King and Penny that, despite a lot of ogling of bimbettes dressed to encourage pneumonia, King and Penny seem much more at home with each other. In fact, the phoniest scene in the premiere found the two of them in their bedroom, each primly and properly tucked into his own single bed, looking uncomfortable.