Hello, it is not such a bad idea, you can build up a business out of this, at least you can try it. There are a lot of people that want to have their own t-shirt, designed by them, and you can make a lot of money out of this, you have just to try it. There are a lot of companies that provide different similar services, there are even companies like https://www.printful.com/start-online-tshirt-business that will learn you how to create your own t-shirt business. Check it out, you won't be disappointed about that.