All kilts get dirty when you roll in the mud, no matter what you do to prevent it. Not a big deal, some kilts are made for this, or at least easy to clean again (waching machine) Utilykilts, Freedom Kilts, USAK casuals, R-Kilts, Stillwater kilts, Frugals etc. The cheaper the kilt the less you have to worry about a stain I guess (you won't get a heart attack anyway).

Other funky option...a cotton kilt apron