This is a bit of chicken and egg, but if you've committed a crime, you are a criminal. If you've been accused of a crime, you are not a criminal, until you are proven to be guilty.
If youve been convicted, you've been found guilty. If you've been found guilty, that means you've committed a crime, and are a criminal.
In order to be an inmate (barring those await trials in jails) you must be a criminal.
Your concern, I think, is stemmed from the unfortunate stigma that follows criminals long after their rehabilitation/incarceration, which is a valid stance. However, if someone is currently in a prison, as an inmate, they must be a criminal in the eyes of the law. That doesn't mean they are criminals from then until the end of time, however.