The key is to provide A) shade and b) wallow.
Shade can be all sorts of things - trees, a "lean to" against their ark, tarpaulin whatever you can provide, they can then be out of their ark, but under cover.
Covering in mud is excellant, but they should do this themsleves by having a wallow - mud about the consistency of emulsion paint. Dig a hole about a foot x a foot and 6 inches deep, fill with a bucket of water, and the pigs will expand it into a full size wallow - just keep adding water.