There's nothing childish about what Mumen Rider does. Monsters are real in his world. They attack cities and murder tens of thousands of people with ALARMING regularity, and heroes are the only thing shown to be able to stop them.
Mumen Rider can't beat most monsters. He knows that, so he focuses mostly on getting people out of the danger zones. He goes into catastrophe, drags people out, and then goes in looking for more people that need help.
In OPs picture, Mumen Rider had 0% chance of beating Sea King. He knew it, Sea King knew it, everybody knew it. But he was the only line of defense that was left stopping a bunch of innocent people and getting slaughtered, so he stood there anyway.
And you know what? He bought valuable time until someone who actually COULD beat Sea King showed up. Even though he couldn't win the battle, Mumen Rider saved a lot of people by delaying what seemed like the inevitable.