Depending on the industry, ugliness may be required. Coupon sites are one such example. One time I redesigned a big well known coupon website (most important pages) and we quickly found out it wasn’t doing as well. People preferred the ugly thing they were already used to.

While there’s no easy way to determine that the pretty styles had anything to do with it, we typically make hypotheses and test them. In fact, large websites and apps are optimized one element or page at a time with small and big tests. The winner is put up against another variation and so on.