When Floyd Mayweather says Donald Trump's "locker-room talk" is A-OK, no one should listen ... CAUSE HE'S ILLITERATE -- this according to "Shameless" star Cameron Monaghan.

We got Cam leaving Catch ... and the actor came at Floyd for claiming Trump's "grab them by the p***y" comment was how "real men speak."

"He's a great boxer, but he can't read! Don't listen to that guy, it's ridiculous."

Cam says real men have respect ... and would never talk like that.

FYI, Floyd's got video proof that he CAN read -- but the kid's got a point.