Diddy -- MOCKS Kendrick Lamar ... You're a JOKE!!!!

The backlash against Kendrick Lamar is in FULL SWING ... with Diddy putting the rapper on blast on Instagram ... and he's even dragging Jay-Z into the attack.

Lamar has been under fire for the past 24 hours for the lyrics he spits out against other rappers on Big Sean's new track, "Control" ... in which KL refers to himself as the "King of New York."

Naturally, Diddy took issue with the lyric ... considering, well, you know ... he thinks HE'S the king of New York.

So Diddy fired back with an image on Instagram, showing himself and Jay-Z, another New York boss, laughing ... along with a caption that MOCKS Kendrick's lyrics.