There are two issues here. One, Mete was not just average among the Canadian defencemen. He was the best player defensively and at the very least was a play driver on offence. If he had played every game I would probably put him as the top defenceman on the team, but I suppose that honour has to go to Timmins. Makar also might have been better if given more minutes, but he wasn't.
The bigger issue is expecting that he would dominate just because he was in the NHL. Mete was a player just keeping his head above water in the NHL. Low minutes, sometimes scratched. A good NHL player would probably dominate this tournament, but a good NHLer wouldn't be released to play at this tournament in the first place. Mete is a marginal NHLer and shouldn't have been expected to dominate. Think of the players released to play for Canada in the WJC this decade. excluding Nugent-Hopkins who never would have been released if not for the lockout: Mete, Virtanen, Lazar, Dumba, Connolly, Schenn. Only Schenn dominated. The rest ranged from good to Virtanen. Disappointment is a subjective thing, but as said it was never reasonable to expect Mete to dominate. Mete played very well, and that was the reasonable expectation.