Do a pop song and stack my Caucasians!
Two possible meanings:
Nicki, by getting on a pop song, is going to make tons of money, stacking bills with white presidents (Caucasians) on them.
In the same way, a pop song will expand her audience to white people (Caucasians), stacking up her fanbase.
To help improve the meaning of these lyrics, visit "In My Head (remix)" by Jason DeRulo (Ft. Nicki Minaj) and leave a comment on the lyrics box