Not mowing your lawn is not really illegal per say by the actual law but cities and towns have codes that you must follow to keep the town and city beautiful and not mowing your lawn can get you a day in municipal court and a possible fine and yes sometimes jail time if you fail to pay those fines or mow your lawn.
Sometimes if you do not mow your lawn and code enforcement comes by and orders you to mow the lawn and you fail to mow the lawn or maintain your lawn the way they want you to then they can fine you and even possibly mow the lawn by way of the city.
Then you'll get billed a heavy fee for the city mowing your lawn.
So if you live in a city or town they can have local codes that you must follow as to how you must keep your lawn mowed and maintained.
So if there are laws in place in your town or city that say you must mow your lawn then you must mow the lawn or face hefty fines and possible jail time.