...I'm gonna ask anyway.
Who'd win the fight?
AFTER THE ANCIENT SPIRITS OF EVIL MADE HIM IMMUNE TO HIS OWN REFLECTION... Mumm-Ra is an excellent sorcerer, plus he has size on his side. A model sheet I saw gave Lion-O's height at 6'9". In The Trial of Evil, we see that Mumm-Ra is head and shoulders bigger than Lion-O. No doubt Mumm-Ra's size gives him a serious advantage.