The difference between a tshirt selling and not selling is whether it makes a statement about the wearer. It doesn't matter how good the drawing on it is, or how clever the words you put on it, that won't make it sell.
Over the years, I and many of my fellow creators have complained that we design something, readers are vocally enthusiastic about it, then no-one buys it. I can think of a dozen times where I laboured over a design and no one wanted it, and as many where 15 minutes work proved wildly popular - because the idea was fine, because it said something that the wearer wanted to say about themselves. There is a huge difference between an image someone likes and an image that fosters the desire to wear it.
Below is a good example of something that makes people laugh but that I don't think says anything about the wearer that they could explain to anyone who asked.
The rules are different for prints, posters and any other designed curio. They're not plastered onto a human body. After I got a button machine, I found that the easiest way to tell if wording worked on a tshirt was to try it on badges at shows. The stakes were low and experimentation was cheap.
Printed tshirts went way beyond saturation point a few years ago; they used to represent the greater part of my income and they certainly don't now. I've used all my easy ideas, so while I can design garments far more nicely than I could in 2003, I (personally) have nothing left to say that I, or someone else, hasn't said. But I try occasionally, because when it works, it feels great. And I've done some fun collaborations where I didn't have to marry my drawings up to some universal notion.
In conclusion: when lizard fights wizard, no one wins. Let that be a lesson to you.