The Q are not absolutely omnipotent, as stated by Q himself in the Robinhood episode, but essentially the Q are omniversal beings who are beyond "mortal" space/time (remember the final episode where Q was putting Picard through his paces at three different points in space/time?
As powerful as Ion is, he would still lose to a fully manifest DC Angel or one of the Endless, rather easily, or even a being like one of the Lords of Order or Chaos. Q is somewhere in those guys level.