Why would it be Assur? You can Daven for anything you want and Hashem always listens. Of course, you’re probably competing against many other people Davening (for both teams) and affecting the game probably comes to Bechirah Chofshis issues (for the players), so it’s probably unlikely that the outcome of the game will be affected by your Teffilos. But the Tefillos never go to waste and HKBH keeps them somewhere for a time when they are needed.
As an aside, I once heard someone joke that the reason the Yankees are historically so good is because they always have the most Jewish and Frum fans so they have a stronger Koack Hat’fillah than any other team.