LL Cool J says Andy Ruiz broke the golden rule when it comes to STAYING successful -- he got comfortable.

If you know LL, you know he's a HUGE boxing fan (see: "Mama Said Knock You Out") ... and it's obvious he was disappointed to see Ruiz blow his run as heavyweight champ.

Ruiz told TMZ Sports he didn't take training camp seriously and "ate everything" -- which is why he showed up to the Anthony Joshua rematch 15 pounds heavier than their first fight.

"At the end of the day when you step in the ring you gotta be ready. And, if youre not ready for whatever reason it doesnt matter, you win or you lose," LL said ... "And he wasnt ready."

"I got respect for [Ruiz]," the rap legend explained -- but he noted, "You cant come into the ring 16 pounds heavier!"

LL knows a little something about sustained greatness -- he's been famous for almost 4 decades -- and he had a little advice for Ruiz moving forward.

"Its the things you do in the valley that gets you to the peaks. And, its the things you do at the peaks that extend the peaks. Part of being great at anything is having the discipline to stay focused when youre successful."

Listen to LL. He knows.

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LL Cool J Drops Truth Bomb on Andy Ruiz, You Blew It - TMZ