About T-Shirt Design
The joke on Pi = Pie T-Shirt is easy to understand, and that is a big part of why it works. It has a clear everyday humor vibe, which makes it easy to picture as a gift or something to wear for fun.
It works for holidays and casual days, but it is also the kind of shirt someone could wear whenever they feel like it. As a novelty t shirt, it has a simple idea that is easy to notice without feeling overdone. This one is mostly about having a little fun with what you wear, which is kind of the point.