It is often hard to discern the meaning of song lyrics, and this one is no exception. You may be correct in giving bish its modern meaning of bitch.
However, bish-bash-bosh is an old idiom that implies giving someone a good thumping, and swish might indicate taking a swing at someone, so "swish, swish, bish" might mean taking a couple of swings at someone and then hitting them.
It may be either, but it needs someone much younger than me to provide a reliable opinion.
North American: Representing the sound or action of delivering a series of blows.
British informal: Indicating something accomplished or completed quickly or simply.
Oxford Living Dictionary