About T-Shirt Design
That's Mr. Asshole To You T-Shirt is funny without trying way too hard, which is probably why the idea works. The everyday humor angle gives it a little extra personality without making the design too busy.
You could wear it for birthdays, bring it out for holidays, or just keep it in the normal shirt rotation. It fits nicely with novelty t shirts that are meant to be fun first and serious never. It works well as a novelty t shirt because the joke is clear and the design has plenty of personality. It has enough personality to be funny without looking like it is begging for attention.