What do you think about UI design importance?

I think it entirely depends upon the context. Obviously, design matters more if you are selling to a design-sensitive crowd (e.g., designers). It matters less if you are selling to a crowd that couldn't care less. That said, I think more people are becoming aware of what is and isn't good design- Design-first companies like Apple have put it front-and-center in the public's awareness. All things being equal, a beautiful and functional UI is the best outcome. There could even be a case for optimizing for beauty first, since users will perceive it as more usable (https://lawsofux.com/aesthetic-u...). There is, of course, a minimum bar for usability under which you cannot go. As someone that does both design and front-end development, I can tell you that it takes the same amount of time to implement a lousy design as a great one. In fact, it might take less time to implement a great design (things like spacing are more consistent and responsive breakpoints are better conceived). The same can be said of coming up with the initial design, although I do think a great design takes a bit longer. This is a long way of saying that I don't think there needs to be a trade-off between aesthetics and usability. I well-executed product can have both at little to no increase in cost or implementation time.