I want to prohibit my daughter from wearing an oversized t-shirt over extremely short shorts in public.
Why? Who is she harming?
She'll be causing the harm to herself
It is good to make sure your child is dressed properly in public. You should protect them from possible perverts trying to sexually assault them.
Don't be silly, nobody is going to sexually assault her while I am there.
But you cannot be there all the time. So it is okay that you want to protect her by telling her that somethings are not okay to wear. If she wore that oversized shirt people would assume that she was not wearing shorts.