Lil Mama Makes STUPID Decision -- Gets Ass Kicked In Court

Lil Mama -- the rapper behind the song "Lip Gloss" -- just got destroyed in court ... after pulling one of the dumbest moves ever ... TMZ has learned.

Mama was sued by her lawyer -- a guy who specializes in intellectual property -- who claims he busted his butt to protect her rights to the name "Lil Mama." Kind of an important guy in her life.

Problem is, the lawyer claims he worked for Mama until 2011 ... but she stopped paying him in 2008. So, he did what lawyers do ... he took her ass to court.

In the end, the rapper (who also starred as Left Eye in the recent TLC movie on VH1 -- suffered an embarrassing and expensive defeat and was ordered to pony up roughly $18k for the bill, interest and other fees.