I'm getting too fucking old, the fucking etymology on this is hurting my brain. I guess I grasp the concept, but wtf does a cake have to do with it. Why can't you just say you find someone oddly attractive or w/e.
The "cake" is a literal cake, and the concept is printed pictures on sticks or straws that you put into the cake as you mention or explain the unconventional person. It's just a "viral" online trend, a random fun thing friends do.
I know a woman that utterly hates that elf dentist guy. She's a lesbian though so maybe this lady is on to something. I need to understand the sexual energy at work here.