Over-centralization has to do with in game statstics, not things like matchups. Going to use two examples, one I won't go into detail with because its not a fighting game, another one because its a fighting game that was so quickly destroyed from its major hype.
Example 1
This pokemon became the CENTER of EVERYTHING pokemon. Its still disputed weather or not he is truly Uber, but never the less, Garchomp took a HUGE amount of focus for every team, so much so that 25 percents of all team had to carry Gengar just to stop him. He overcentralized the game, not because he destroyed so many pokemon, Blissey does that and you don't hear anyone crying, but because the game became far to CENTRALIZED around him.
But that isn't a fighting game so I'll go here, and by here I mean the only charecter to take one of the most hyped fighting games of all time and take it off the competative scene within a matter of 3 months flat.
Now, here are some facts about V-13
1. The 2nd best character in the game.
2. Had even matchups on a good deal of the cast, only hard countered 3 people.
3. Despite being drastically overcentralizing, there were still charecters that would pop up in the chart from time to time, most notably, Mike Z playing Mike Z. . I mean Iron Tager.
4. Most charecters outside of the top tier had harder matchups. V-13 was not their worst matchup.
NEVER THE LESS. V-13 turned that game into competative **** because of how much she over centralized the game. Now that game is a laughably small side event at Evo while everyone waits for the new version of the game to come out, the new version being the only reason V-13 wasn't banned, as tournaments had basically boiled down into V-13s taking all top spots, and Mike Z.
Does this sound familiar at all?