I think, to a degree, there is maybe an unspoken perception or even expectation that the captain not only has good leadership abilities but in general is the best player on the team, offensively. We all know that's not the case, but there's more than a few NHL teams whose captain is that type of player.
I would have no idea what happens in the dressing room and whether Tavares has the confidence of his teammates. From an ambassadorial perspective, he certainly fits the bill in terms of projecting a sense of maturity, being even keeled, and keeping his nose clean on and off the ice. From the sponsors' perspective, he is a great spokesman. But a great spokesman does not make a captain.
I know previously being captain of the Islanders may mean diddly squat, but maybe that was just the path of least resistance for the Leafs. Find someone who had that role before and have them take it on, whether on an interim basis (even for a few years) or permanently. Tavares certainly had much promise in the OHL and being first overall draft pick, lots of expectations. Has he achieved them offensively? I would say no, insofar as meeting the potential fans and the league had. But has he been a solid journeyman who has been a decent player? I'd say yes. That doesn't make him captain material either, but I don't think he is, by any stretch of the imagination, the worst captain. That, I would ascribe to Dion Phaneuf, who should have never been in any leadership position in the first place.
Certainly, fans have had a good treat in past years with good solid captains like Sundin, Gilmour, Clark, etc. I would even say Ramage was decent. Would I rank Tavares in that bunch? No, but he is not the worst. In my lifetime (so far) the Leafs haven't really had a lot of bad captains, Phaneuf excepted.