Yes, he could. The only truth of any consequence that follows from this is that if he did, then there would be a task he could not perform (viz. lift the rock). But that is no limitation on omnipotence, because performing such an action (i. e. lifting a rock that he can't lift) would be logically impossible, and the logically impossible has never been taken (except in the case of Descartes) to fall within the scope of omnipotence.
An another instance of the same phenomenon concerns God's inability to change the past. There are all sorts of actions God can perform, of course. But once he has performed, he cannot make it such that he has not performed them because doing so is logically impossible. Thus the particular case of the stone God cannot lift presents no special difficulty here.