I guess it looks so bad because it is a mediocre interpretation of the UI fads of seven years ago. This is why it is always a good idea not to follow the latest fad, especially if you have constrained development resources. Unfortunately, KDE 5 seems to be headed into a similar direction, copying the latest "flat design" fad, in a rather uninspired fashion. Until KDE 5 is ready, this fad will already have passed.