It’s not necessarily more important, but it does come first. Backend is the spine. You can get more profit with a relatively unimpressive backend, but a fantastic front-end(of course you’ll be bleeding money in serverside costs). Many people underestimate the importance of a good user interface and saving users time or making things comfortable for them.
On the other hand, if your backend is really poor, the whole thing just doesn’t take off. So with a good backend and a meh UI, you can still make some decent money, but without a decent backend there’s no product. The conclusion would be that the UI is a profit booster, not a maker or breaker of things like the backend is. And yes, if the UI is atrocious people just won’t buy your product, unless you’re the only one on the market offering it.