Are Physical ailments commonly mistaken for mental illnesses is this a rhetorical definition?

No, it is not a rhetorical definition.

A rhetorical definition is a definition that is used to create a specific effect or to persuade the reader of something. It is often used in advertising or propaganda.

The statement "Physical ailments commonly mistaken for mental illnesses" is not a rhetorical definition because it is not trying to persuade the reader of anything. It is simply stating a fact.