From a technical perspective, he can't be a main tank. The sole reason for this is that he lacks a shield (or a defensive tool that functions equivalent to a shield). Main tanks are heroes like Reinhardt and Sigma, as their chunky shields allow for high defensive capabilities for both themselves and their allies. Roadhog can't defend his teammates in the same capacity. Furthermore, he's also quite vulnerable himself for the same reasons, so he's best as an off-tank. However, if you have no tanks on your team, and you want to play Roadhog, then by all means, please do. One off-tank is better than no tank at all. Just prepare to die more than usual.