My opinion on the matter, as someone who loooooves her collection of nose-thumbing tees:
Cheeky t-shirts have their place, if thoughtfully chosen. Humor can grease the public social skids for our children, especially those with visible disabilities and/or limited communication skills. If your child has the language skills to have a conversation about the shirt, even better. But it's important to not underestimate our children's impish sense of humor.