The TagMeNot tag is free. The original QR code design is released with a Creative Commons license (CC-BY) license, so you can use it as you like, as long as attribution to TagMeNot.info is preserved .
You can download the tag and print your own t-shirts, badges, buttons or have it stitched wherever you want. You can even sell your own TagMeNot tagged objects. In this case let me know if you use online stores for distribution, so we can arrange their listing on this website.
You may know that QR codes can be modified to some extent: you can add colors, or embed other images. Consider that doing so may reduce the readability of the tag and therefore its functionality.