I just discovered that using the Wild Hunt Transform memento makes you immune to mudballs. The character still physically reacts to being hit by a mudball if idle, but will not appear to be covered in mud. If on horseback or using an emote after using the memento, you become fully immune.
I'm not a big fan of mudballs, so this comes in handy. Spammers will get bored and go mudball somebody else lol