We don't have any sources saying that Aristotle was Alexander's tutor until the Life of Alexander by Plutarch (AD 49-119). We have other sources for Alexander's life (e.g., Flavius Arianus and Quintus Curtius Rufus) but they picked up later in Alexander's life, after Aristotle ceased being his tutor.

Much of what Alexander did, Aristotle would not have "approved" of. Aristotle is against empires, for instance, and clearly values the independence of Greek political societies (polis). The Macedonian conquest, carried by Alexander's father, goes against these values.

But there are so many qualifications that we need to make.

The first is that Aristotle is from northern Greece, namely, Stagira. He doesn't exactly fit into the Athenian milieu. Some people say that when he heard of Alexander's death, he fled Athens, sensing (correctly) that now the Athenians were going to revolt against the Macedonians who were keeping watch on the Athenians. What I mean is that Aristotle probably didn't fit neatly into the camp of people who had been "defeated" by the Macedonians.

(That being said, Stagira had been destroyed by Philip in 348 BC, which means that Macedonians and Stagirites weren't on each other's good sides. However, it's also important that Philip later re-built Stagira apparently as thanks to Aristotle for tutoring Alexander. Perhaps this won Aristotle's affection? We'll never know.)

The second is that we don't know when in Aristotle's life he developed the views in the Politics that would bring him into disagreement with Alexander's actions. Plutarch says that Aristotle taught him ethics and politics, among other things. But why even think that tutoring someone in politics means teaching him mainly or only what you think? Perhaps Aristotle taught Alexander Thucydides and Plato. Plutarch tells us that Aristotle's tutoring made him value Homer's Iliad so much that he slept with it under his pillow. As I said, we don't even know whether Aristotle had developed his characteristic political views, or any, by then. Some scholars argue that the Politics is actually two treaties put together later, with one part being a very early work and the other being much later. Some people say all of it is quite late. We just don't know.

My experience teaching undergraduate students Aristotle has led me to think that some people simply think that "tutoring" him means more than it has to. (Students often blow this out of proportion — as if Aristotle were being hired to make Alexander into an “Aristotelian.”) Aristotle could have been hired merely to walk Alexander through the classics, so to speak, and not make him a disciple.

The last point is that philosophers such as Aristotle were very open to disagreement. I am not convinced that just because Alexander went against the values that Aristotle espoused in the Politics, Aristotle would have hated him or been disappointed in him or been angry with him. Aristotle was part of a vibrant intellectual community where disagreement was the air that you breathed. Aristotle wrote whole books about arguments with other people, such as the Topics and Sophistical Refutations. Moreover, by the time he was hired by Philip in the 340s BC as a tutor for Alexander, surely Aristotle would have known what he was getting himself into. Macedonian power had been growing for a long time, and there were powerful voices (e.g., Demosthenes) speaking out against it. We can conclude what Aristotle's thoughts on the matter were from the fact that he accepted a tutoring position there. I doubt he was surprised when the father of the man he tutored made a play against Athens and company. But we can't know Aristotle's reactions, beyond that, because of the lack of evidence.