There are several ways in which to answer this question.
First, if you are referring to the Elders, the ones who oversee the whole community, then you can definitely say that the mood is one of constant alertness. They monitor everything in this dystopia world. In a very real way they control the world; in fact, they have engineered it.
Second, if you are referring to the members of that society, then we can say that constant alertness does not apply. In fact, they are dulled and so they just exist. They know little and their minds and emotions are controlled.
Third, if you are referring to the audience, then constant alertness is a good way to describe the tone. On the surface things seem perfect; in fact, things seem too perfect in an eerie way. When Jonas becomes the new receiver of memories, it becomes clearer that things are not well. There is suspense and this suspense makes the audience alert.