Ordinarily, Orthodox people have sobriety and nepsis. Therefore, they don't just smile at the drop of a hat like most Americans think is proper. So strangely is American culture skewed towards foolish grins and empty smiles, that it's happened on this site that there would be a picture of some mirthful Eastern Europeans, and they were thought to appear somber.
There's nothing unusual, bad, or undesirable about the way the people are looking in that Palm Sunday church pic. Haven't they been fasting for weeks, emaciating their flesh and depriving themselves of comforts and beauty sleep and keeping vigil? Well if not, they need to find better reasons to look depressed.
As one monk at Jordanville once told a bright and cheery young American novice: "Be Russian! Be depressed!"