It doesn't really matter in the context of Sisyphus being happy or not. There could be any number of reasons for his fate, things going on in the rest of the world, etc., but he's pushing a boulder all the same.

I would guess that Camus is probably also applying a Western Christian lens on the mythology - that is, Sisyphus' fate is an eternal punishment that he must continue to do forever. By making this thread, you have probably put more thought into the context of the analogy than he did.