Is it safe to masturbate while you have dermatitis?

Probably: Unless you have dermatiotis on your penis and masturbating makes it worse.

Depends: Is the dermatitis on the hands or on the penis? If you have an inflammed penis, you should make sure to avoid infection and worsening dermatitis. You should wash well and use liberal lubricants. You should wash well afterwards and apply appropriate topicals and or medications. Why the dermatitis. First see a doctor and get treated. Avoid things causing the inflammation. Use common sense.

Sure: If it's on your special places and it's sensitive, vary your technique to what is comfortable.