Pigs don't have sweat glands, they have to add moisture to their skin in order to get it to evaporate and take away heat. And mud has moisture in it. It sure does. Otherwise it would be dried dirt.
Among the many insults hurled at pro-Muggle witches and wizards (such fruity epithets as “Mudwallower”, “Dunglicker” and “Scumsucker” date from this period) , was the charge of having weak or inferior magic.
Von Kaganick writes: " We suddenly have to deal with the most frightful adversary, the autumn rain and mud which the Russians call 'Rusbotisa', endless mud so gluey and sticky that it sucks and holds on everything."