No, this would not be considered a legal order. A DNR order proper is an order issued by a medical practitioner (at least in Maryland, this is a doctor, an APRN, or a physician's assistant), and has to take an extremely specific form (either a specific state-issued form, or a specific state-issued device like a bracelet). A "DNR" shirt can't serve as a DNR order, and so EMS will ignore it.
With physicians, it's more complicated -- they have to obey your wishes whether or not you filled out the state form, and so if you clearly express a wish not to be resuscitated they will issue a DNR order. However, they can't do this unless it's clear that you're giving informed consent to the order; wearing a shirt isn't considered to show that you made an informed decision that you intended to be binding. An advance directive (specifying what care you'll want if you aren't conscious, which is the only time a DNR applies anyway) will require some way to confirm that this is really what you intend; generally, this consists of a (witnessed) signature or oral instructions to a practitioner.