I think it's really about not keeping your audience bored, but not being too awkward either. You need to be able to react to the game in some way, though the reaction very much depends on what's happening and your own personality and commentating style, and it's good to talk to the viewer, when there isn't necessarily anything to react to in the game, and I wouldn't say it necessarily has to be about the game, although it can be and might be better in some cases. Of course, if there's nothing to talk about of the game, it's better to just talk about some topic else than the game like how you're doing and what you think of recent events or anything like that. Some games are more action-packed and have a lot going on and in some games you kind of have to entertain the viewer more.