Sex sells, and it always has.
Album cover designers have used women (and men, if you’re a beefcake collector) to sell records as long as there have been records. In the pre-internet days, the local record store was as good of a place as any for a guy to catch a cheap thrill.
But what qualifies a sleeve as “cheesecake”? Some collectors draw the line at pin-up like album covers in a mid-20th century style. I agree that those are the top of the heap (of the cake?), but to my eye that definition is too limited. Entire genres and decades are immediately disqualified by such a narrow categorization. On the other hand, “any album cover featuring a pretty lady” is way too broad.
I guess the answer is that you know it when you see it. What I think qualifies as cheesecake you might not, and that’s cool. It’s not like there’s some sort of Olympic rules governing record collecting.