Would it be considered crass of him if he were to applaud? Well, he does regardless, because he's never been too concerned about appropriate behaviour, even under the best of circumstances. And Arthur is just too much fun to mess with! So if it bothers him to see Prussia applaud all those twists and dives and rolls, then so much the better.

Because he really doesn't know how to act around Arthur without being a pest, and at least being annoying gets him noticed.