Does good design matter to a product's success? Or is function more important?

As a developer, I can say that you may have great functionality, but the user will not understand how to use it. And he will leave without appreciating the full power of the functionality. That's why it is always better to call a designer in conjunction with a developer )

The more appealing it is, the more better it is.

Marketing is very important.

of course functionality is important but design is the most critical and key factor of success. It is such hiding treasure. If you don't show treasure, it doesn't make sense for users!

Definitely, it does matter. The first impression is always the most important. If the design is unattractive, the risk of losing a client is very high.