Do not cut a gardevoir’s dress! I apologize, anonymous, but what many people often perceive as being a dress are actually skin flaps designed to protect a gardevoir’s fragile legs. Cutting these skin flaps may result in severe bleeding, and it will certainly result in intense pain.
That having been said, what you can do is ensure your gardevoir is well-hydrated and help her acclimate by providing her with plenty of shade until she adjusts to Alola’s tropical environments. You may also be able to clip a gardevoir’s dress back by using soft bands or ribbon to gather her skirt behind her, much like a ponytail. Alternatively, you may also wish to teach her ice-type moves, such as Ice Punch or Icy Wind. Generating either will create a short blast of cold air that she can summon at any time to cool herself off.
But really, the most important thing to keep in mind is hydrating, as when it comes to that at the very least, a gardevoir’s body works the same as a human’s: the more water she drinks, the easier it will be for her to regulate her temperature.
Best of luck, anonymous!