The super sentai concept itself (like a lot of toku) is wacky: people in colored suits fighting monsters,which become big, and the heroes then fight them with giant robots.
With that concept, even the most serious sentai (like Timeranger, GogoV, Changeman, Jetman) keep a wacky side . Besides, in every sentai, there are completely silly episodes. Moreover, usually, the villains are completely over the top.
Seriousness in a sentai series is a relative concept. Dairanger has a silly side, but is nonetheless more serious than Zyuranger, Carranger, or even Boukenger.