Is Benadryl (diphenhydramine) good for a face rash (acne cream allergy)?

By mouth, possibly. Rash to a cream usually is due to contact dermatitis. The best treatment is avoidance, namely stopping the offending cream. If the rash continues after stopping, usually a steroid topically or by mouth may be required. Benadryl (diphenhydramine) by mouth will reduce itching, but not necessarily affect the rash. You better see a dermatologist.