Design isn't just what something looks like, but how it works. So, good design is indeed a mirror—it reflects the brand and values of a product, company, or person; it also reflects the audience themselves or a value that means something to them. Bad design is just visual trickery meant to give the illusion of greatness. But, true greatness comes from something that is usable and valuable, not just from something that looks nice.
Since it's in my wheelhouse, let's talk about good versus bad digital experiences. A site or app with good design is searchable, navigable, and usable. It provides value to those who use it. It gives them information, inspiration, or helps them solve a problem in their life. It can also have a lovely aesthetic and some technological innovation that surprises and delights... but those things are just icing. A site or app with bad design can look beautiful, can use all the latest technological tricks, but can still be a bad design because it falls short on function.
People aren't coming to a site to see how pretty it is, they are coming to a site to accomplish some goal. Good design facilitates that, while bad design falls short.
Let's say you are trying to buy an airline ticket, for instance. You haven’t bought any online before, so you don’t have your go-to site. You Google "airline flights" and see some choices. You go to two different sites—one that looks cool and on-trend and one that looks basic and dated. You decide to try the cool one first, assuming that since it looks good, it will work well. But, you can't find the dates, times, or destinations that you're looking for, or it isn't easy to update a search once you've started (you have to start all over for instance), or maybe you find the flight you want without trouble, but something goes awry when you try to actually pay for it. (Hint: you'll know you're dealing with a bad design when you feel frustration starting to set in.) All of these issues are flaws in the design. So, you go to the second site—the one that is a little dated (looks like its logo and colors were last updated years or decades ago). But, it works great. You find flights right away, it lets you update your search on the fly, and checking out is a breeze. Do the aesthetic flaws matter to you anymore? Nope. You're happy that it worked and you go on with your life.
Now, obviously, some marriage between these two is ideal. You want your site to function well so that it's useful to your audience, but there is definite value in it being on-brand, up to date, and cool—not only because you want to reinforce your brand in every design choice you make, but because people will judge your product or company on how it looks (especially when they are comparing you to others and they don't have much to go on besides looks... as in the airline flight example).
Moral: don't blow smoke; make your design reflect a true value for your customers.