Assuming that a t-shirt might show not just texts but also graphics…
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I have chosen only short phrases because I think that they make better t-shirts, but feel free to mention longer sentences if you think that it would make a good t-shirt. 
When I read the title I immediately wanted to suggest “Party like it’s 1999!”
(Maybe it could also say “4th law of robotics:” before this line?)
“I designed the O-Ring!”
I thought about adding that bit but it would make it more TWP-specific, while the more generic phrase works also as a reference to Prince.
I’m sure that a t-shirt like this would be perfectly interpreted as a reference to a videogame.
“The kids at school will beat you up with love!”
‘How may I be a-boo of service?’
That would work especially well in public-facing jobs. Or in the future when there are Thimbleweed Park themed restaurants.
That sounded nice until I remembered that they would have to serve S&D Diner quality food in order to be authentic 
S&D diner food is excellent. That story in the Thimbleweed Nickel about botulism was a smear job.
Oh my, I would throw up if I…you know…still had a stomach!
Oh yeah, darn. “I’ll have the salad.”
“Autographs are $100.” Ransome themed
“Hey dude, what’s your damage?” Teddy themed
And on the back it could say: “OK, $50.”
this is the only one that can be understood by (and seem funny to) people who haven’t played TWP.
I would say that “beep you!” could also be understood by other people …
yes but it would not be funny without context. I think.
In the US they use the beep in TV shows and movies. So I would assume that most people will think that is a reference to these beeps.
Ok, but if it’s only a reference to that, where’s the joke? That joke is only an in-joke. (oh, wait)
Because proudly putting something on your T-Shirt yet still restraining yourself by using a written beep is funny. It contradicts your expecations.
Oh, I got it now. Yes, makes sense 
Maybe a catchy phrase about toilet paper?