If not, provide a fan for the stalled mule. Make sure that clean, fresh water is available always. While trailering, try to haul in the cool part of the day. Make sure trailers have adequate ventilation. When stopped, open windows to allow breezes in the trailer and don't let the mule stand in a non moving trailer very long.
Temperatures can rise quickly inside a non moving trailer.
Gradually condition your mule for the work you will be asking him to do. Always allow him to warm up adequately before a training session or performance and to cool down adequately after working. No matter what the humidity, however, temperatures nearing 100 degrees are always dangerous. There are mules who colic. There are mules who founder.