A good design is one that sells a lot and a bad design is one that doesn't sell. This is a business, so nothing else matters. Oftentimes I make designs that I like but sometimes I make designs that I personally wouldn't buy because I know it would sell. Some of my most successful designs are ones I love, and some of my most successful designs are ones I wouldn't buy or wear. But I think I tend to have better luck on average with designs that I personally like a lot. I think part of this is because I have very mainstream taste. I love the summer blockbuster movies and I hate those much less successful little dramatic movies that the critics love.
Inexperienced people have a lot of wrong ideas about what is a good design. Their opinions on designs often differs quite a bit from what actually sells! N00bs know nothing about what sells and often are extremely off-the-mark when they come up with design ideas. I had a lot of pretty bad design ideas when I got started, but I've seen many other n00bs come up with far worse stuff than I did. That's why I laugh when a fresh beginner says "I have a lot of great ideas for designs." Sure you do. Don't bet the farm on it.
After having sold a lot of different designs and also having had a bunch of designs that didn't sell at all, I have learned a bunch of things about what really sells and what doesn't sell and the reasons why and why not.
Also, the more experience you have, the better your intuition becomes about what's going to be a good seller and the more often you are proven right! In the beginning, understanding what sells is extremely difficult, almost impossible. That's why you need experience.
Sometimes making designs that you really like is a successful way to make designs that sell. But not always. It depends on how many other people share your taste. If you are very in tune with a huge number of people and you come up with a hot new idea that you really love, you might have a huge success on your hands. If you are very out of tune with a large number of people, then that could spell poor sales for you.