Conway is basically Rubio, a robot who says nice things at the right time but without any charisma, conviction, or thought. The type of "perfect candidate" that media elites cook up. The writers of HoC also bought into this brand of candidate being successful. I think the actor was fine in portraying the type of politician he was intended to portray.
Of course, in real life, guys like that flop and no one likes them. I think that explains why many viewers thought Conway was the result of poor acting. I think the character himself was a poor actor, intentionally so. The disconnect is that the HoC writers believe, like the media elites, that a candidate such as this would be popular, but that's not the actor's fault.
Underwood is like a Bill Clinton or GWB, a great charismatic figure who connects on an emotional level, through hope or fear. Of course Conway would look terrible going against someone like that. I don't think it's the actor's fault. It might even be intentional.