A funny graphic t shirt is all about humor. And humor is an expression that everybody knows and can relate too.
Each culture and society has its own set of jokes, funny stories and events. Most humor is made from these things in these cultures.
If its current affairs that are going on in the world, then that's a great place to start with humor.
It would be funny yes, but for Microsoft it wouldn't be.
Now lets say you had a comical t shirt with a broken window on.
If you were to show it to small town that didn't even know who Bill Gates was, they would probably think your broken window t shirt was a business for repairing windows. (they wouldn't see through that now).
Funny can also be offensive at times.
An image to one person might not be appropriate to another person. If I had to show a t shirt with a funny picture of four elephants squashed into a mini-car, we might chuckle at it, but to person who loves animals that might just be a sensitive area.
I do think that most graphics today, especially people who have exposure to media like TV, internet and cellphones can identify and relate to a common humor.
But in all honesty, if you create designs that you enjoy and others around you can laugh at then make it and wear it. Even if it a private joke that friends will enjoy.
So go on humor me with your funny graphic t shirt.