Who says Captain Marvel is the World's Mightiest Mortal? The guy who wrote the nickname? Superman is the Man Of Steel, he's not actually made of steel. Plus, Superman might be immortal. And therefore exempt from the list of Mighty Mortals.
And Superman does beat Captain Marvel in Kingdom Come. He's bleeding but he's also gripping Batson's face with his hand. If he squeezes, his face implodes. Superman wins.
Marvel has the strength of Hercules. Thor is probably equal to or greater than in terms of strength level compared to Hercules. In JLA/Avengers, Superman knocked Thor on his ass. Ergo, Superman beats Captain Marvel.
And JLU sucks? Wow. You must not know anything about what makes the DCU great, since if you did, you would understand that JLU is the greatest example of comics to tv there ever was.