Does high frequency treatment make melasma worse or better?

HIFU is probably safe. However, UV radiation is not for melasma. 
Try a topical regimen like Melarase and also one of two of these oral supplements for pigmentary control: Scler-X PIH Relief Complex from VenaSmart or Pigmentation Correction Complex from KareSkin.
You will need wide spectrum UV protection with SPF 30+ sunblock as well. 
H Karamanoukian MD FACS

Depends on the wavelength. I would avoid UV, however high frequency ULTRASOUND will not worsen melasma. 
Melasma can not be completely cured, however effective treatment are possible. This is because your skin is extremely sensitive to UV and even the smallest amount can stimulate your pigment cells to produce colour. The mainstay of treatment is strict UV protection- hats, sunglasses and SPF every 4 hours.

I prefer to use a combination of creams and laser for melasma. Hydroquinone can be used (5-8%), along with vitamin A creams. I usually start my patients on laser (low dose Q switch or Picosure) a few weeks after they commence on creams. Another laser I use is the Clear and Brilliant in the Permea setting combined with creams.

In some cases I combine glycolic AHA peels, and in other cases I use a tablet called Tranexamic acid to help. Melasma treatments should be tailored to each patient.

All the best, Dr Davin Lim. Laser, surgical and aesthetic dermatologist. Brisbane, Australia