Pros get blasted for acting like the judges are wrong, now they're assholes for acknowledging that they're correct? Isn't that exactly what a pro is supposed to do? Are they supposed to delude their celebs?
You're conflating different things. It's entirely possible to be critical and supportive at the same time. It wasn't what he said, it was how he said it, and the body language he was giving off at the time. Aly is a champion-level athlete, and there's no way you get to that point without being able to take a lot of criticism. But Mark wasn't being critical, he was being exasperated. When you are a teacher, there's a time to tear down, and a time to encourage, and that was very, very clearly the latter.
What Mark should have done: he should have told Aly, It's my fault for not helping you with that more, I should have known better. Because actually, it is his fault. He is a professional dancer, and she is not. Maybe they spent the entire week working on her flat feet only to have them pop up again. Maybe he really was exasperated, and maybe with good reason. It doesn't matter. He's being paid to be supportive, and he was not.