Some would argue that the question should be, "Are there rules to fashion design?" but the concrete presence of one inarguable rule powerfully suggests the likelihood of others. That one rule is, "If you don't sell your designs you won't have the money to make more." It doesn't matter what the press says, it doesn't matter what the industry says, it only matters what your potential customer says. Yes, or no. That has to always be your guiding principle if you want your primary income to be from your own designs. Relax, there's a lot more freedom hidden in that stone statement than there first seems. The good news is that the solution lies in your art itself. It's your understanding of one vital fact about your designs that will propel you forward. Let's delve into the role of the principle of, "good design" in sales. The first thing I suggest to fashion designers is to replace the word "good" with the word "effective". An "effective design" is one that pleases your customer enough to not only buy it but to keep them coming back season after season to see what you're up to. If you have that you have a business, if you lose it you're done. Effective design as defined by your customer is what you need to learn. I'm not telling you you need to turn into a non-artistic service robot, but if you don't completely understand why your customer buys you you're going nowhere.
Once you have Rule #1 carved into your brain you will start seeing Rule #2: It's what I have in common with my customer that makes them buy me over others. What's important to understand about Rule #2 is how much subconscious communicating you do when you design. Almost every designer I know has had only around two to three years to catch on to who their customer is and why they buy. That razor sharp truth has been true for some people who had pretty deep pockets as well as those who only had lint in theirs. The critical thing to grasp is that there are many other people who are attracted to the same forms and fabrics you are. There are a substantial but finite number of visual elements that humans find pleasing, thus there are shared elements in effective forms of artistic expression. Not all people find all visual elements attractive. To some Rei Kawakubo's play with shapes is powerful art while to others it is silly and grotesque. But she sells her work and that is the only thing that matters. Enough people share in what interests and attracts her for her to have a career. Her fashion designs can then be defined as effective. Effective artists create work that resonates with many people. They express ideas and feelings in ways that many find pleasing. It's shared aspects and the values that they signify to your customer that create the viable market for your designs. The key then is to discover the common elements that you & your customer find attractive and pleasing, as well as what values your brand embodies that they believe in. Go forth and figure it out.