It would not be illegal to wear just a diaper and T Shirt outside but some places may have dress codes such as in stores, malls etc.
Those places can make you leave and tell you to put some clothes on over the diaper if you are allowed to return to those places.
It's unlikely you'd get into trouble walking around town in just a diaper and shirt but it's still not the right thing to do.
You may get made fun of, have pictures taken of you, could get beat up etc.
And it's not a good thing for most people to see nor does everyone want to see a teen or adult wearing just a diaper in public.
Heck I don't even like to see babies or toddlers out wearing just a diaper as I think it's tacky and we don't want to see the diaper.