When first considering this dilemma, i wanted to say no just based off his head shape. His huge nose would obstruct any path of the vagina because his mouth is at the bottom of his head.
After giving it some more thought i found you could rotate Phineas’ head to remove the nose from the equation. but anatomically it seems unnatural to put his neck in this position
Luckily for isabella, this can be fixed by phineas laying down with his head propped up. the angle rotation faces his mouth upward, making it geometrically possible for him to eat pussy.