MMA superstar Chael Sonnen is a HUUUUGE fan of Donald Trump -- both personally and politically -- but says it's time for his friend to do something no one expects ... forgive Kathy Griffin

Sonnen tells TMZ Sports ... Kathy's gruesome photo shoot with a beheaded, bloody Trump face definitely crossed the line -- and her attempt at shock comedy failed big time. 

But now that Kathy has apologized and admitted she messed up, Sonnen is encouraging the President to accept it ... saying it would be a "really cool move."

Sonnen says he knows Trump won't take his advice -- but he makes a strong case.