Well, Incineroar is a starter with a very low speed and mediocre defenses, therefore you would want a move that's strong enough to finish opponents with one usage majority of the time.
That being said, Earthquake is a much better option to use than Outrage.
With Earthquake, you do not have to go through the trouble of using the same move over and over for 2-3 turns - which Outrage provides. With the use of the Outrage, it's only super-effective type against is Dragon, which is just normal-effectiveness against Incineroar. Also, if Outrage is used, you run the risk of the opponent possible switching out to a Fairy-type, since the move is continuous for 2-3 turns (very predictable). However, even if the opponent doesn't switch out, you will turn up confusing yourself after Outrage has fully executed, thus jeopardizing Incineroar in exchanging health for a useful turn.
With Earthquake, it provides a super-effective counter to Incineroar's rock-weakness. Without it, you would find yourself relying on simple STAB moves and mostly Jolteon/Blastoise to be effective defenses for Incineroar, making it almost unnecessary in your team. Furthermore, Earthquake does double damage against opponents with Dig, and can be quite useful in double/triple battles with Flying-type/Levitate allies.
Also, even if your up against a Flying opponent, there's only roughly 15% of flying opponents who are Dragon, or Dragon opponent's with the ability Levitate - thus not provided great chances of Outrage serving more purpose than Earthquake
Your Choice! Hope I helped! :)