How bad is slight or mild schizophrenia?

Not Sure. There is no diagnosis of mild schizophrenia. For schizophrenia of different types, there is a wide range of symptom severity. Generally, schizophrenia can be very treatable and patients with schizophrenia can be functional and productive.
Not sure of ? It's tempting to say being "slightly" schizophrenic is like being "slightly" pregnant; but, unlike the latter state, some people do have relatively mild symptoms, sometimes for years, such as hearing voices occasionally or having "flights of fancy" that go way beyond normal. They may stay stable or improve, but typically the condition worsens with time. Bad mainly because it can get worse.