Technically you wouldn't be breaking any laws since your private parts would be covered up when wearing the adult diaper outside.
I have a fenced in yard and sometimes I like to just go outside and wander around the yard in my underwear but I wouldn't do so if my yard wasn't fenced in.
Not because it would be illegal but just out of respect for the neighbors and it wouldn't be nice for neighbors kids to see me in my underwear.
But since my yard is private no one can see me in my underwear so I do so.
If your backyard isn't fenced and there's kids around then I would not wear just the diaper in the yard because someone may call the cops on you even though technically you wouldn't be doing anything wrong.
But I would recommend putting up a fence around your yard before going out in just a diaper just out of respect for neighbors.
I used to live next door to a guy who was incontinent and during the summer he would be outside on his patio sitting on the patio wearing nothing but a shirt and his diaper and it never bothered me although he got hassled sometimes by some other neighbors.
He wasn't bothering anybody and was minding his own business and was a really nice neighbor.