Can someone please tell me what the 'M' in MNC stands for?
It stands for "Mythical" because it's not a true championship. You can't have a real championships based on opinions of which team is better. Without a tournament of some kind, it's all just guesswork.
Another reason some use "mythical" is that several teams (Notre Dame, Michigan, Minnesota, etc) count "championships" from back in the dark ages of college football when there was no universally accepted body or group that handed out titles. Michigan claims eight national championships prior to World War II when there was no universally accepted method of determining who was champion. With such diverse claims to the title - not to mention their esoteric nature - it's hard to say who was or wasn't "the champ." Hence mythical.