The Ganon vs IC's is impossible.
Assuming the Ice Climbers know desyncing and chain grabbing, Ganon is doomed. Ganon hits hard, but how can he hit them when he is camped by Ice Climbers and being chain grabbed to death?
The ganon ICs MU wouldn't be 100:0, that's terribly unrealistic. I'd say somewhere between 90:10 and 99:1 but I have no idea where.
In Brawl, there's no actual 'unwinnable' MU by direct definition of the word. There are, however, a huge amount of match-ups that are so incredibly difficult to win that it borders on ridiculousness. For instance, Zelda vs. Olimar is a ludicrous match-up. Even from an outsider's perspective (which I am), it's pretty easy to see that Zelda just can't do anything that a good Olimar couldn't, under normal circumstances, punish or completely avoid punishment himself (ie taking damage) - his Pikmin outrange her and shield him, he out-camps her (reflect is okay but it doesn't protect against everything all the time and Oli can afford to just keep throwing more), he wins close range, his aerial ability outclasses her.... etc.
However, when I say 'normal circumstances', you have to realise that these are actually very rare. To have totally normal circumstances, the player has to be thinking logically and quickly and has to be in the right position to counteract the moves made by the opposite player without confusion and nerves. In tournaments, heck, even in friendlies, this just isn't the case. People get nervous which affects their ability to play the game effectively and just having your fingers in the wrong place on the controller can mean you screw up a defensive or offensive option you were planning to use. For instance, a good example here is the Marth vs. PK kids match-ups. I'm not sure how long you've been here or whether you've had ample opportunity to learn a bit more about a variety of characters in the game, but to clarify, this MU is a -3 for both PK kids (which is a large disadvantage). There's a lot behind it but one of the biggest reasons it's not at least -2 or lower is the chaingrab. Lucas and Ness both suffer from an extra 10 frames of grab release, meaning they can get grabbed again by certain characters before they literally have a chance to do anything. Marth is one character that can heavily abuse that fact. However, even though in theory it's a near-infinite chain grab, plenty of times a Lucas or Ness player will manage their way out of the process because the Marth messed up his timing due to nerves and the opposing player capitalised on it. This type of situation (nerves) has quite possibly the biggest effect on the Ice Climbers because they can chain-grab everyone in the game from 0-death (using a different method) if they input the commands correctly. But even 9B and Vinnie (two of the best ICs players in the world) regularly drop chaingrabs halfway through due to incorrect inputs and nerves. It's just how tournaments and things work.
So no, in my opinion there is no truly unwinnable match-up because player control is a variability and will always be one (I don't see an instance where anyone would ever become good enough to mirror a TAS CPU), so theory can be different from the reality. It's just that even with player variability, the disadvantages do exist and can be capitalised on and as a result that character will usually find it very difficult to win against a player that can abuse at least some of his advantages.
