I don't know how much the term is used outside the States, but traditionally over here, it means a woman whose main quality is being sexually attractive in a showy way.

So in the original cartoon Velma is the brains of the outfit, but in fan-made art and cartoons, she usually seems to be reimagined as little more than a sex object, pretty much the same as Daphne.

I’ve seen her turned in to a ‘cheesecake’ character

From the Cambridge Dictionary - Cheesecake:

cheesecake noun (WOMEN)

[ U ] mainly US old-fashioned slang photographs of sexually attractive young women wearing very few clothes, or the women who appear in such photographs

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beefcake informal