no
Rocky Marciano retired undefeated. Joe Louis had almost 70 wins in his career. Even in MMA there are many fighters with a lot of great fights and relevant wins through a bigger timespan. Fedor, Silva, GSP, Aldo, Dan Henderson, Big Nog, Werdum... speaking of Werdum, the guy is a multi BJJ champion and was HW champion. He also beat Fedor.
Conor crushed Aldo and that was historic. other than that, he didn't do and won't do much to ever be considered something so insane as greatest of all time in all combat sports. There are many fighters everywhere who have more respect for their craft than Conor. He is a creature of his time, of money over honor and social media bullshit. He barely beat Nate Diaz and lost to him too. But he is gonna fight Floyd, so people forget about it. Maybe he throws a punch that connects and Floys gets knocked out, but so what. One fight doesn't make anyone the greatest ever. It's really shortsighted to think that.