first off was it actually in the yard or in the grass between the sidewalk and street? ( I can tell you Some municipalities have ordinances about parking in the grassy area between the street and a sidewalk ) If not was a tire on or part of the vehicle blocking the sidewalk ? or a fire hydrant close by?
If none of those apply I would Fight the ticket , when the judge asks you about it, ( politely) tell them you want to fight it because the vehicle is legally registered, was parked on your lawn which is considered ( your) private property and was NOT blocking a fire hydrant, the roadway, or a public sidewalk. and as far as you know there was no ordinance or laws broken ( in fact you should ask him to explain what law or ordinance was broken if none of the above apply )