I'm afraid I can't say much. The rash could be viral, it could be associated with sweat and clogged pores, and I suppose it might be, at least in part, the seborrheic dermatitis you suspect. It surely isn't a steroid allergy, but I see no advantage to her suing the steroid cream until you see the dermatologist, since it doesn't seem likely to help much.
That said, I don't think this is going to turn out to be a serious or long-term problem. While waiting for her appointment, your wife can use an over-the-counter itch lotion like Benadryl or Sarna.
Best.
Dr. Rockoff