When mashing apples with a rotary masher, the mash and juice gets quite oxygenated.
Does this contribute to the acidicity of the raw apple juice?
Oxidation has the opposite affect. It creates hydroxides.
What oxidation will do however is impart a candyish jolly rancher like flavor which I personally find distasteful.
A typical and well characterized example of what this ends up tasting like is comparing a good strawberry wine or watermelon wine with those that have oxidized. Good wines of these two taste like the fruit, oxidized ones taste like those jelly filled strawberry candies (for strawberry) or jolly rancher candy (for the watermelon) and it's disgusting in a wine.
That's why we use ascorbic and/or citric acids when preparing fruit, it greatly inhibits and reduces oxidation of the fruit.