Emperor Evulz and his machinations were always in the background; he's just now taken center stage.
Alternatively, the series only becomes serious whenever Emperor Evulz is around. Each time he leaves or is defeated, the series goes back to its lighter tone until his next apperance (assuming there even is one).
While frightening and dangerous, the work does its best to remain humorous and lighthearted regardless of Emperor Evulz's machinations and appearances.
Emperor Evulz is more dangerous and threatening than the other villains, but he is still rather comical and goofy.
Justified: Because of how peaceful the world was until then, the protagonists were unprepared to face serious threats, and as a result are completely frightened and devastated by the arrival of the unbelievably heinous and cruel Emperor Evulz. Before Evulz came along, the heroes only had to deal with minor problems that would only cause minor inconvenience if not solved. But now that Emperor Evulz is the antagonist, the heroes face problems as serious as genocide and corruption.
Reconstructed: Emperor Evulz's arrival is a taste of the reality of life outside the show's Sugar Bowl. The main characters understand this, know better than to panic, and end up leading the struggle against him.
Back to Knight of Cerebus. You know, the site was a lot more playful before that trope came around...