About T-Shirt Design
The best part about Being An Adult is the Dumbest I've Ever Done T-Shirt is that the joke feels natural instead of forced. Fans of everyday humor will probably get the joke first, but it is simple enough for almost anyone to understand.
You could wear it for casual days, bring it out for holidays, or just keep it in the normal shirt rotation. It has the same easy attitude people usually want from sarcasm shirts without sounding forced.
The graphic tee style keeps the whole thing casual and easy to understand. The joke is quick, the message is clear, and that is really all it needs.