I think a couple things I've made are shirts now, and totally without my permission. Feel free to do the same if you are so inclined. I don't really know how I would react if I saw someone wearing a shirt with my stuff on it, I think I would be embarassed and hide.
You should deffinately own up to it~ X3 If the person likes the meme enough to wear the shirt, theyd probably shit a brick to meet the creator. Think of it as being famous...... for something few really care about.