I think the drinking is just from the fact drinking goes on at halloween parties and isn't a necessary part of the costume.
Okay, but then what makes the costume so great? 
It would be funnier if the Octomom's kids were drinking, wouldn't it?
You need to let go of the drinking and realize it isn't part of the costume.
It's visually amusing. I also find it amusing that because of Angelina's baby collecting fetish, you could sell this as a packaged costume to anyone and it would work, regardless of race.
Angelina's baby collecting fetish
HA!
Well said, raz, she really does have a baby collecting fetish.
Gramps
Not sure I get the joke. She has a lot of kids, but why would they be drinking?