About T-Shirt Design
You do not have to think too hard about My Imaginary Friend Thinks You Have Serious Mental Problems T-Shirt. The joke is right there and it gets the point across fast. The everyday humor angle gives it a little extra personality without making the design too busy.
It makes sense for casual days or birthdays, especially if the person already likes this kind of humor. People who buy t shirts for the joke as much as the shirt itself will probably get what this design is going for. The joke is quick, the message is clear, and that is really all it needs.