I never say never because we are all individuals but ....... in over 10 years of working with Thermage and several thousand treatments, we've not had a patient who's melasma became  worse.  Also, I've never seen this reported in the medical literature.  Thermage is a radiofrequency technology (sound waves) not a laser (light waves).  Many lasers can and often do worsen melasma so you're wise to be careful.    You didn't mention that you are double sunscreening but I'm going to assume that you are.    Dr. I