The online market for swag with stuff printed on it is massively oversaturated right now. The get rich quick grift economy has moved from drop shipping electronics to drop shipped printing.

There are a bunch of online print shops that will take an image file, print it, and ship it to a customer for you. There are also data services that scrape Etsy, eBay, etc, then sell sales data to other sellers. So even if you make a unique design that sells, competitors will steal them and be selling them for less almost immediately.