The capital B is just to make it seem like being a bitch for the reason of being a bitch is cool.
NOTE: a girl PMSing or having a bad day is just that. Bitch with a capital B is a girl (maybe even a guy) who is just always sour unless you kiss his/her ass.
GIRL: Bitch with a capital B!
The girl on my bus is a self-declared Bitch with a capital B.
Me: being a bitch isn't cool. It's just stupid.
CapitalB: It ain't cool unless it's Bitch with a Capital B!