From a monotheistic point of view Arceus is not god, calling it a demiurge or something of the like of that is fine, but it is not god.And just because Arceus supposedly created the universe, that does not mean it is the supreme being, it is not infallible, it does not possess infinite knowledge or infinite wisdom, nor is it infinitely kind. And there may never a god Pokemon from this point of view.

From a Polytheistic point of view Arceus could be seen as a god, but no more so than the other higher legendaries.

If we are to assume a polytheistic viewpoint then Arceus would be the creator god, but no more, seeing how little control and influence it has over the other Legendaries. Arceus has no control over the physical plane, rather other Legendaries such as Dialga, Palkia, Xerneas, Yveltal, Rayquaza, etc. And if one goes by the movies, they can lash out a cause catastrophic damage, with no sort of influence from Arceus whatsoever, and Arceus itself is prone to go into a destructive rage, a rather ironic thing for the being of creation to do. Even if one disregards the movies, the Pokédex entry for Giratina still has the implication that such mighty beings can grow violent and smite the world. Additionally Giratina resides in the distortion world, a realm that it alone controls, where the physics of this world are altered.

So within the Pantheon of Gods perhaps Arceus is up top, but if it is, then it either does not care to rule over the other Legendaries, or it is does not possess the ability to, which instead belongs to an even higher being that resides beyond even it.