What is the moral of the story?

I think that themes are more relevant than morals in a book like this. Consider the theme of "who we are".

When the reader looks closely at the main characters, the reader might note that "who they are" equates to flawed beings. Miranda's parents are broken up and distancing themselves from her, Justin's parents don't seem to care about him, and Jack's parents struggle financially. Even Julian, the rich kid with everything, is flawed. His parents don't teach or model empathy. In fact they are bullies themselves.

Auggie has an obvious flaw that people see. Others have less obvious flaws such as Justin's nervous tics. Acceptance of oneself must occur so that acceptance of others follows.