Why is poor grooming part of the diagnostic criteria for schizophrenia? Is it different from poor grooming found in other disorders like depression?

Poor grooming. Along with delusions, hallucinations, disorganized speech & behavior, and negative symptoms like affective flattening, part of the criteria for schizophrenia include social/occupational dysfunction. Self-care can be part of this, and lack of it can show in poor grooming. Depressed people may also have poor self-care and/or grooming, but their overall symptom picture differs from schizophrenia.
Not necessarily. Impairment from a mood disorder or a psychotic disorder can prevent people from self -care. It is an indicator of function.