It is legal for an adult to wear just a diaper and shirt and shoes in public.
However while it's legal for an adult to wear just a diaper and shirt and shoes in public it could make you a target for bullying and someone might call the cops on you if they are uncomfortable.
If you do wear just a diaper and shirt and shoes in public, you also must avoid kids when possible and avoid seeming like you're drawing attention to your diaper around kids.
That part could get you into trouble, even if you're not actually intentionally trying to expose yourself to kids.
I would personally never wear just a diaper, shirt and shoes in public as I would also not have the courage to do so.
But I do go outside in my fenced yard that is privacy fenced wearing just a diaper or just a diaper, shirt and shoes.
I get so envious though of toddlers that go into stores and public places wearing just a diaper or diaper, shirt and shoes and they are not embarrassed of course.
Just yesterday I was in Dollar General and a mother brought her 3 year old son in the store wearing just a Pampers Baby Dry diaper and shirt and his shoes and he was riding in the cart and smiling and talking to others and not a care in the world about being seen in his diaper.
I wish that were me and wish I could do that, but sadly I can't.