LeBron James -- I Suck In 'Trainwreck' ... But the Movie's Awesome!!

LeBron James turned into "Rotten Tomatoes" right in the middle of Hollywood last night -- giving a street review of his new movie "Trainwreck" ... and ripping himself in the process.

James was leaving Le Jardin nightclub ... when he told us the movie is unbelievable -- but "I'm not that good in it.

LeBron, you're being hard on yourself.

Our pals at TooFab.com saw a screening of the flick and tell us, "LeBron is FUNNY!" Provides really good comic relief and plays really well off of Bill Hader."

"I don't think he sucked."

By the way, inside the party LeBron was dancing his face off while Future performed for the pumped up crowd. We got video.