Is a 9 or 10 cost minion hit by Freezing Trap unplayable?

You cannot have more than 10 mana at once, so the only way to play a card more expensive than 10 mana is to reduce its cost to 10 or below. So, no you are not missing anything. The way to work against hunter traps generally is to deduce which trap it most likely is and play around it. In case of freezing trap it's using an expendable minion (or one with a good battlecry) to trigger it or use a card which destroys secrets (e.g. eater of secrets)

However, I don't see how this is overpowered. The likelihood of this situation occuring is fairly low and if it happens, you know it is a possibility and have to play around it. There is also the possiblity, that the hunter played misdirection and your own C'thun might have hit you in the face. Taking those two traps into account, it's an easy decision not to attack with C'thun until you have another minion to trigger the trap available or a card to destroy the secret, as the risk is simply too high. So unless, you are in a desperate situation anyway, there is little reason to take that risk.

Additionally take into account what other trap he could have played. Playing an explosive trap, when you only have a C'thun on board is utterly pointless, unless you'd die from the damage; the same applies to dart trap. Bear Trap only makes sense, when the hunter survives the hit from C'thun. Snake trap pointless as well, unless you want to trade, which is unlikely when using C'thun. Snipe? What's the point with a C'thun on board? Apart from that freezing trap is by far the most common hunter secret. So, you can see, unless the hunter was utterly desperate, there was little reason for him to play anything besides freezing trap or misdirection.

Yes, a 9 or 10 mana minion bounced back by a Freezing Trap becomes unplayable. There's (currently) no way to get above 10 mana and Cost-reducing effects for minions are very rare: as of today only 3 minions offer a cost-reducing effect, Emperor Thaurissan, Pint-Sized Summoner and Summoning Portal. Of these only Thaurissan sees play, since the other two have usually no immediate impact on the game state and can be easily removed. Also, in this specific case, Thaurissan would still require to be played after your C'Thun has been bounced back in hand AND survive for at least another turn after he's been played.

The "Play a low cost minion, hope it doesn't die and attack with it first the next turn" is exactly the way to play around Freezing Trap. Usually anyway the Freezing Trap is used way before turn 10 and most of the Hunter decks in the current meta use only one copy, so normally the chance of your C'Thun being bounced back is pretty low.

The perceived Overpowerednees of the trap is offset by the fact that it has to be played proactively, so the Hunter doesn't get the chance to target the minion to bounce, giving you the opportunity to play around it.