Depends on the swaptmeet, but for the general ....No- most swaptmeets are played out and only sell the same thing produce, socks, meals out of bags of chips, fake sunglasses and 5 t-shirts for $10.
Back when easy-e was alive swap-tmeets were the rage with casette tapes, and thus good t-shirt venues.
Now if its not bootleg Elmo, Spongebob, Justin Beaver, Hollister or Lakers, then most people wont buy your shirts.
Don't get me wrong. you will sell and get a lot of foot traffic, but not enough to send you home with a boner.
There is a few night swaptmeets with concerts, but still you need good assortment.
The more styles, colors, designs, sizes you have the best chances you will have.
Try your luck at Fairs and Specialty Events.. all you need is Ca Sellers permit and its free,,
Whos your audience...Rockabilly, Punk Rock, Indie, Rap, Reggae, Vegans, Pet Lovers, there are many more places where you can sell to "your crowd", rather than selling to a vague market who is only JUNK SHOPPING.
Best of luck.