You don't necessarily have to beat an all time great fighter to become an all time great fighter. The fact that Mike is the youngest ever to win the heavyweight championship, went on to unify the titles, was the most famous athlete on the planet (more famous at the time than Jordan), and to this day arguably considered to be the hardest hitting fighter and consensusly hailed as the most exciting means he's not overrated. You don't attain all those accolades if you're merely overrated.