Katy Perry -- Taylor Swift Is a Hypocrite … When it Comes to Girl-On-Girl

Katy Perry just weighed in on music's biggest bitch sesh -- calling Taylor Swift a full-on hypocrite for accusing Nicki Minaj of taking down a woman.

Perry thinks it's hysterical Swift told Nicki ... "It's so unlike you to pit women against each other." The joke, to Katy ... is that Taylor went for her neck with "Bad Blood" -- and proceeded to make a ton of cash off the song that's all about throwing shade at Katy.

Nicki took notice -- and favorited Katy's tweet.

Taylor might wanna call for backup.