Depends on the type of crime.
Crime to vehicles is very low. Most vehicles around here aren't worth it.
I do not believe violent crime is terribly high. It seems to come in spurts, starting randomly and ending due to the diligence of Mansfield PD.
It appears much of the crimes are committed by people who know the victim, or even live with them.
In my experience, you cannot leave anything unlocked. Don't leave a car unlocked, window cracked or the classic Mansfield plasticwrap over a broken window.
It used to be that you would feel safe walking at night alone anywhere in the entire city. That is almost still tue; however, when those random burst of crime flare up... this is almost always someone "new" moving into the area, and a month or so later, you will be treated to the sight of Mansfield PD. It's usually a group of the same people, and Mansfield is too small for them to hide.
Mansfield is a safe place with some diligence on your part. You are far, far more likely to have a run in with a heroin addict than anything else.